Talk to me about lenses?

[sjt269]

Hey all,

I'm considering getting back into photography after a few-year gap while I've been at uni. As a teenager, I was pretty into photography and I had a Fuji bridge camera (which I still have) that got a lot of use, but now that I'm earning money, I'm considering going for an entry-level DSLR and learning photography properly. I'm pretty much set on the Nikon D3100, though I'm in no rush and I'm waiting for the price to come back down to the ~£250 it was a few weeks before Christmas. I can't afford the D5100 at the moment, though I'll keep my eye out for any not-to-be-missed offers.

The thing that's throwing me though is the step after buying the camera and the kit lens - buying additional lenses. I know this is a while away for me, from a purely financial standpoint at least, but I'd like to know what I'm buying into. I've got a vague working understanding of shutter speeds, apertures and focal lengths, and I've been doing a lot of reading lately, but never having used a DSLR or a specialised aftermarket lens I'm still left puzzled by a few things, so I hope you good folks here can help me out with a few things.


i) Why have an 18-55mm and a 55-200mm lens when you could have one lens that does 18-200mm? I picked those numbers purely as an illustration, but I don't see why you'd want 2 lenses to cover a range of focal lengths when one could do it. Is it price, or is it a case of jack of all trades, master of none?

ii) Carrying on from that, I don't understand primes. They seem to be fixed focal length lenses (or is there more to them?), which I take to mean they have no zoom capability. Again, why would you want a 50mm f1.8 prime (which I think is one that's mentioned a lot) over a lens that covers 18-55mm and on the face of it seems more versatile? I can only guess that the performance of the specialised lens at 50mm must be a lot better than the non-prime lens, but I don't quite understand how. Does anyone have any examples of pictures taken at the same focal length on a prime and non-prime to highlight the advantages of a prime lens?

iii) Again, fixed focal length lenses. The lenses say things like '50mm f1.8' or 40mm f2.8G', which seems to imply fixed focal length and aperture, but I see people on the forum here talking about using a variety of apertures with these lenses. Am I misunderstanding forum posts? Is the aperture stated in the spec a limiting value, rather than a fixed value? Why then do some lenses state a particular range of apertures?

iv) What's meant by 'fast' prime?


I know these are all noob questions, but I'm at the stage where every new thing I read confuses me further! Any replies will be much appreciated.

Thanks,
Steven

 

[Raymond Lin]

i) Why have an 18-55mm and a 55-200mm lens when you could have one lens that does 18-200mm? I picked those numbers purely as an illustration, but I don't see why you'd want 2 lenses to cover a range of focal lengths when one could do it. Is it price, or is it a case of jack of all trades, master of none?

Hi.

1 - You got it right in your question, jack of all trades, master of none. Same principle as prime is generally better than a zoom that cover the same focal length (there are exceptions of course).

ii) Carrying on from that, I don't understand primes. They seem to be fixed focal length lenses (or is there more to them?), which I take to mean they have no zoom capability. Again, why would you want a 50mm f1.8 prime (which I think is one that's mentioned a lot) over a lens that covers 18-55mm and on the face of it seems more versatile? I can only guess that the performance of the specialised lens at 50mm must be a lot better than the non-prime lens, but I don't quite understand how. Does anyone have any examples of pictures taken at the same focal length on a prime and non-prime to highlight the advantages of a prime lens?

A prime is better for a few reasons

1 - It is generally sharper.
2 - It can let more light in (we call this faster) This in turn drastically can affect your photos. Light is the key to a good photograph, understand light will go a long way improving your images.
3 - Bokeh, this is what is know as how nice is the blurry bits in the photo. General population don't really pay attention to this, not consciously. They just see blur, but subconsciously, I believe that when someone see a photo with a nicer bokeh, it makes a nicer photo. And a nicer lens generally gives a nicer bokeh.

I don't have a 18-55 or a 50/1.8 so I can't show you examples, although the bokeh from the 50/1.8 isn't that great in the grand scheme of things. The reason people love it is because it does give a lot of bokeh compare to the 18-55 due to the aperture.

iii) Again, fixed focal length lenses. The lenses say things like '50mm f1.8' or 40mm f2.8G', which seems to imply fixed focal length and aperture, but I see people on the forum here talking about using a variety of apertures with these lenses. Am I misunderstanding forum posts? Is the aperture stated in the spec a limiting value, rather than a fixed value? Why then do some lenses state a particular range of apertures?

Think of a lens as a metal tube, which technically it is. It's purpose is to let light through.

To focus the lens has to move glass backward and forward inside to focus. Then the image narrows down through the glass and it lands at the back of the camera into the film/sensor. Like how light pass through you eye call into the back of your eye into the retina.

Aperture are like a variable opening inside the tube that controls how much light go through. The way to control how much light go through comes in 2 ways.

1 - size of the hole
2 - how long for

It's like opening a tap to fill a bucket. The bigger the tap, the faster the bucket gets filled up and the less time you have to turn it on.

What also affect this is how much light (like how fast the water is coming out of the tap). It is your job as a photographer to make sure that bucket is filled to the right amount, over it you get an over exposed image, under fill it, you get am under exposed image.

So, the numbers.

A 50mm/1.8 lens....get a ruler out.

If a 50mm lens is 1.8. 50/1.8 = 27.77

That number is the size of the opening.

So a 50/1.2 has a 41.66 mm opening.

A 200/2.0 has a 100mm opening, it is a BIG piece of glass !!! (thus the price, see?)

People say they use a different number is because they don't have to use the 1.8 or 1.2. They can stop it down (close it up), why? As above, to let less light in, for correct exposure. The other reason is to control the depth of field. This is how much you see.

You look at the computer screen in front of you, do you notice that on the edges of your vision, things are out of focus? Have you wondered why? It is because your eye balls as an approximate aperture of around 2.1 (when i darker rooms). And you know how you squint things are more clearer? That is like the pupil closing up, so if a lens closes its aperture, more things come into focus.

iv) What's meant by 'fast' prime?

A fast prime is generally referred to a prime lens with a smaller aperture number, normally under 2.8 (2.8 would be a fast zoom, zoome as a lens that covers more than a single focal length). Although depending on focal length too. As above, the focal length/aperture formula? If they make a 1000mm Prime Lens at 1.0 aperture, it would have a 1 metre wide opening....how on earth are you going to pick that up? That is the stuff of NASA, not really for us mere mortals.

So a 50/1.0 is possible (you can get 50/0.95, it just means the opening is bigger than 50mm), but the rule of thumb, normally lenses under 2.8 are fast.

 

[hbuzz]

Hi Steven,

I'm not a Nikon man but should be able to help on the lens front.

There are several reasons you may choose to go for two lenses covering the same focal range as opposed to just one. The lens trying to to cover those ranges such as a very wide angle (18mm) to moderate zoom (200mm) will more likely be compromised and suffer more distortion and vignettering (darkness at edges of frame, more of a problem with full frame sensors 35mm than smaller sensor sizes). The zoom lens covering the least range will also more likely be sharper that that of a similar priced lens trying to cover more.

Prime lenses are indeed a fixed focal length however they are NOT fixed in their aperture. Prime lenses are usually faster than zoom lenses, ie they stop down to a smaller aperture ie 1.8 however this actually means that the physical hole for light to pass through the barrel of the lens is wider allowing for more light to reach the camera sensor which is a good thing. This is good for low light photography enabling you to use lower ISO speeds and faster shutter speeds (good for sports) which is why they are referred to as "fast lenses". A cheap prime lens will be far sharper then a cheap zoom lens. A cheap prime lens for instance the Canon 50mm 1.8 (about £80) can rival and pro zoom in terms of sharpness and still stop down further. However the drawbacks are that you can't zoom and will have to move about more to compose a shot. Some purists prefer this method and it can help with making you think about composition.

There are various websites where you can view the results from lenses against each other, one off the top of my head is pbase. I recommend investing in good glass as it's exactly that - an investment which will last years.

 

[sjt269]

Fantastic, thank you both! I think that may take a few read-throughs, but that's exactly what I was after.

Bokeh, this is what is know as how nice is the blurry bits in the photo. General population don't really pay attention to this, not consciously. They just see blur, but subconsciously, I believe that when someone see a photo with a nicer bokeh, it makes a nicer photo. And a nicer lens generally gives a nicer bokeh.

I've seen a few comparisons of this recently while searching for what 'bokeh' meant, and you're right that I've never consciously looked at the blur before, but looking at two almost identical photographs taken with different lenses, the difference is quite striking. I didn't realise primes were particularly nice for this though.

Light is the key to a good photograph, understand light will go a long way improving your images.

I like to think I have a pretty good understanding of light (I'm a theoretical physicist), though it's on a rather more microscopic and mathematical level than is useful for photography!

So am I right in saying that the same f-stop value at two different focal lengths corresponds to a different physical aperture size? So a 50mm at f4, say, isn't really directly comparable to a 100mm at f4?

And on the subject of zooms, is it right that longer focal length = more zoomed in, and if so, is this always true or just a rule of thumb? I'm more used to telescopes, where the magnification is given by the ratio of focal lengths of the lens (eyepiece) and the telescope itself, but I've never seen camera zoom described in quite the same way. In the case of the telescope, I can see that the magnification has a simple relation to the lens focal length, so is this simplicity also true for zoom lenses in cameras?

hbuzz, you've mentioned full frame and crop sensors there, so I'll take the opportunity to ask this question too - what's the difference, and what's the effect on the focal length? I keep seeing qualifications when talking about lenses on full frame cameras as 'equivalent to...' some other focal length on a crop sensor camera. I'm also under the impression that crop sensor cameras are the 'cheaper' normal cameras, and full frames are the elite top few. Is this right?

 

[Raymond Lin]

Think that the human eye is about a 45mm lens (people say 50...but it is closer to 45).

So a 1200mm lens is very much zoomed in. 500mm lens is less and 100mm is less still.

In general though, something like a 24mm to 100mm can cover 90% of what I shoot. If you shoot lions in the wide for example, a 24mm will get you killed, so you want something "longer", and more zoom.

When people or advertising describe 10x zoom, they divide top focal length over bottom.

So 10x zoom could be a 10 to 100mm lens. Or 20 to 200mm or 4 to 40mm. The problem is that with a small compact, 4mm isn't that wide....When I said 24mm lens here, I am talking about one on a Full Frame camera, traditionally a 35mm film/sensor camera. The term "Full Frame" is not full to any extent, it is just a description.

Anyway, a 1200mm lens, a prime lens, mathematically is a 1x zoom. But as you can seem it can see something much better than a 10x zoom from a compact.

As for boken quality, it is really lens dependant.

 

[Nexus]

Think that the human eye is about a 45mm lens (people say 50...but it is closer to 45).

I actually looked into the explanation of this a while ago and its not actually the human eye iris that is similar to a 50mm lens, its actually the perspective that the lens creates.

A photo taken with a 50mm lens creates a perspective of things in foreground and background very much like how we see them in real life. Where as a wide angle is giving an exaggeration and a telephoto can compress the background etc.

 

[Raymond Lin]

lol, yes, perspective, the field of view.

That's what they are referring to, not the built quality of the 50mm lens obviously.

http://www.cambridgeincolour.com/forums/thread106.htm

The image focal length of the human eye is approx. 22 mm, but drawing direct conclusions from this about the angle of view is more problematic because (i) the back of our eyes are curved, (ii) the sensing area (retina) is quite different than a digital camera and (iii) the resolution is not constant throughout (with greatest acuity at the center and gradually less towards the edges), amongst others. Each eye individually has anywhere from ~120-210 degrees angle of view. However, the dual eye overlap region is somewhere around 130 degrees-- corresponding to a a near fisheye lens focal length on a digital camera.

However, for evolutionary reasons the extreme perimeter of our vision is really only for sensing motion and large-scale objects (ie, the lion suddenly approaching you from the side). The central angle of view of say around 40-60 degrees or so is what most impacts our perception of a scene. Subjectively, this would roughly correspond with the angle over which you could recall objects from a scene if you had kept your eyes in the same position. Incidentally, this is pretty close to a 50 mm "normal" focal length lens on a 35 mm camera sensor (43 mm to be precise), or a 27 mm focal length on a camera with a 1.6X crop factor.

http://www.photographymad.com/pages/view/standard-lenses

The technical definition of a standard lens is one whose focal length roughly matches the diagonal or the film or image sensor. For a standard full-frame 35mm camera this gives a focal length of around 43mm.

In reality, the actual focal lengths chosen by manufacturers tend to be slightly longer than this. For a 35mm sensor, 50mm is the most common standard lens, although some companies do sell lenses which are closer to 43mm focal length.

 

[hbuzz]

It's lenses when used with a crop body where the focal range is referred to being equivalent to that on a "full frame" body not the other way around. The lens focal lengths are based on the 35mm sized sensors which was the standard consumer film cameras which carried over to the digital age. For example if you have a lens of 17mm-40mm lens, attached to a 35mm body it will give that focal range. However attached to a crop body we usually can multiply that figure by 1.6 (dependent on camera).

If I attached my 17-40mm lens to a crop body it would appear when looking through the viewfinder to be approximately that of a 28-65mm lens which would be a nice walk around lens, on a 35mm body it would be more suited to wide landscapes and not good at all for portrait work. What is actually happening is that we are taking a crop of a smaller area of the center of the lens if you imagine a 3:2 rectangle and place a smaller rectangle within it this will be the crop sensor. There are advantages and disadvantages of crop bodies. They often have faster fps (frames per second) as there is less mirror to move repeatedly which may be good for sports/wildlife. They also have less of a problem with vingnettering as the edges of the frame are closer to the center of the lens. Disadvantages can be poorer low light performance with increased noise in a photo.

Hope that helps!

 

[D.P.]

The focal lengths given are the actual physical focal lengths of the lens design and is completely unrelated to the 35mm film size. A 100mm lens will focus its field of view onto a plane 100mm away.

he reference given t FF cameras is really just because everyone got used to what those focal lengths looked like on film cameras, when digital cameras came out, especially with tiny sensored point and shoots no one would know what a 6-17mm lens would act like. But saying that was a 28mm - 80mm lens made sense to people swapping over fro film.


My point is simply that the focal length given in mm is a physical measurement, it is like saying the lens weighs 600g. Doesn't matter if the lens is designed for a small sensor or large format film, that is just a physical property.

 

[sjt269]

So you need to know the sensor size of your camera to be able to work out exactly how the lens will act? And do different non-full-frame DSLRs tend to have wildly different sensor sizes, or is there some standard they all adhere to?

 

[Raymond Lin]

Each manufacturer make lenses for their cameras, whether it is the bigger (full frame) or smaller (cropped) sensors.

Canon call these EF and EF-S.
Nikon call these FX and DX.

With Canon you can use all EF lenses on all cameras, but you can only use EF-S lenses on cropped cameras. On the camera mount you will see a red dot, line that up with the red dot on the lens barrel when you mount it. EF-S lenses has a white dot, so line that up when you mount it.

So if you have a cropped sensor camera, and a bunch of EF-S lenses. Then you decide to get a FF camera, you will need a new set of lenses. Or you can just only get EF lenses to being with.

I think Nikon lenses is similar, I am not sure 100% how that work though so someone can explain that part.

 

[King Damager]

Just to clarify, because its something I struggled with (I skipped crop so it doesn't matter, but anyway), I feel it's important to point out that a 100mm doesn't suddenly have the zoom capabilities of a 160mm on a crop. It's your field of view that is adapted. So your field of view goes from being 100mm to the equivalent of a 160mm on a FF. It does tend to mean you get better megapixels for that field of view, but MPx is something I generally avoid for comparisons. I'm not saying they're not important, but the sensor is much more important for IQ...

It's really weird to explain, and I'm yet to read a very good explanation, however just be aware that you don't suddenly get 60mm extra zoom on a crop... Otherwise all wildlife togs would be using them...

Raymond, I think you can mount DX lenses on some of the newer FF bodies, but I can't remember... I vaguely recall you can switch the modes on some so that they can do that..

kd

 

[Raymond Lin]

If you mount a 24mm on FF and mount the same FF on a cropped camera.

The cropped camera will feel like it is looking through 35mm. However, you don't get a 35mm image (technically 38.4mm), you just get a 24mm image with the edges trimmed off. Any lens distortion (in the centre) will still be evident, where as if you use a 35mm you will get less distortion.

The real world use is that generally the centre of lenses don't distort too much so you can use it this way. Plus it is typically the sharpest part of a lens too so it can be pushed like this.

 

[King Damager]

Yeah, that's pretty much what I meant

kd

 

[Energize]

Raymond, I think you can mount DX lenses on some of the newer FF bodies, but I can't remember... I vaguely recall you can switch the modes on some so that they can do that..

An APS lens can be used on a FF nikon in either DX or FX mode as the user chooses, by default the camera switches to DX mode when a DX lens is mounted.

 

[D.P.]

Just to clarify, because its something I struggled with (I skipped crop so it doesn't matter, but anyway), I feel it's important to point out that a 100mm doesn't suddenly have the zoom capabilities of a 160mm on a crop. It's your field of view that is adapted. So your field of view goes from being 100mm to the equivalent of a 160mm on a FF. It does tend to mean you get better megapixels for that field of view, but MPx is something I generally avoid for comparisons. I'm not saying they're not important, but the sensor is much more important for IQ...

It's really weird to explain, and I'm yet to read a very good explanation, however just be aware that you don't suddenly get 60mm extra zoom on a crop... Otherwise all wildlife togs would be using them...

Raymond, I think you can mount DX lenses on some of the newer FF bodies, but I can't remember... I vaguely recall you can switch the modes on some so that they can do that..

kd

Erm, I think you are confused a little. Nothing changes to the lens physically but a crop sensor is exactly that, it crops the center of the image that would cover a FF sensor, effectively giving you more reach.

The trick is pixel density of sensors. Crop sensors often have a higher pixel density that their FF sensors, therefore you get more pixels per duck than the same lens you on a FF sensor given the same lens. With the d800 this has at last changed. The d800 and crop d7000 have the same pixel density, therefore a lens used on either will give the same pixels for the given subject at a given distance, but the d800 will have a load of surrounding pixels. In software you can crop the d800 image to get something which is almost pixel for pixel identical to the d7000 image.


Due to the higher pixel density of crop sensors they are indeed commonly used for wildlife and sports. Canon used to have an entire line of cameras that were
Higher density crop sensor with a 1.3x crop.
In the days of the nikon d3 and d300 many people used a d300 for wildlife and sports.

 

[King Damager]

Erm, I think you are confused a little. Nothing changes to the lens physically but a crop sensor is exactly that, it crops the center of the image that would cover a FF sensor, effectively giving you more reach.

The trick is pixel density of sensors. Crop sensors often have a higher pixel density that their FF sensors, therefore you get more pixels per duck than the same lens you on a FF sensor given the same lens. With the d800 this has at last changed. The d800 and crop d7000 have the same pixel density, therefore a lens used on either will give the same pixels for the given subject at a given distance, but the d800 will have a load of surrounding pixels. In software you can crop the d800 image to get something which is almost pixel for pixel identical to the d7000 image.


Due to the higher pixel density of crop sensors they are indeed commonly used for wildlife and sports. Canon used to have an entire line of cameras that were
Higher density crop sensor with a 1.3x crop.
In the days of the nikon d3 and d300 many people used a d300 for wildlife and sports.

I did understand, just my explanation was poor xD

kd