Lenses and focal lengths on a APS-C camera

crazyswede

crazyswede

crazyswede

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I am a bit confused (Still learning all this camera stuff) about lenses and my Nikon D5300.

I am interested in the following wide angle lens:

Sigma 10-20mm f3.5 EX DC HSM Lens for Nikon Digital SLR Cameras with APS-C Sensors

For my D5300 it says....Effective angle of view: Nikon DX format; focal length equivalent to approx. 1.5x that of lenses with FX-format angle of view. (http://imaging.nikon.com/lineup/dslr/d5300/spec.htm)

Does that mean that if I get that Sigma lens the 10mm becomes 15mm on my camera (Also the 20mm becomes 30mm also with that lens)?

I have read that the 'Samyang 24 mm F1.4 Manual Focus Lens for Nikon AE' becomes 36mm because it is for a full frame camera. This sort of makes sense, but the Sigma is for an APS-C camera which the D5300 is, so is that different?
 
Basically yes

The focal length itself doesn't change but due to the way the smaller sensor works with the lens, the effective focal length is 1.5x more.

If you're looking for buy something like a 10-20mm lens then it doesn't matter too much since that's about as wide as you get without going fisheye.
 
In DSLR, AFAIK, all lenses are advertised in 35mm equivalent, doesn't matter if the fit is for APS-C or FF, it is the equivalent length if mounted on 35mm.

So in essence, any advertised lens that fits your APS-C Nikon needs to have a 1.5x on it due to the sensor being smaller than 35mm.

"Crop sensor" is another term used if that helps.
 
crazyswede said:
So does that mean that the 18-55mm lens I got with the camera when I bought it is really 27-82.5mm and the other lens I had which is 70-300mm is really 105-450mm because it is cropped due to it not being FF?
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Yes, but in my experience there's no point wrapping your head round 35mm equivalence while you're shooting. It's best just to understand what 18mm and 55mm look like on the camera you're using, not a hypothetical camera with a larger sensor that you're not!
 
crazyswede said:
So does that mean that the 18-55mm lens I got with the camera when I bought it is really 27-82.5mm and the other lens I had which is 70-300mm is really 105-450mm because it is cropped due to it not being FF?
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Yep pretty much.
Just remember that your actual focal length is as stated on the lens, the 'effective' focal length is the effect of the sensor size on that focal length.
 
Knowing how the crop/ff focal lengths look can be useful but mostly you're better off worrying about how it all looks on the body you have.

There's obviously limits of FF/Crop. A FF will be able to go wider and crop sensors have an advantage with telephoto reach (how much reach is debatable though depending on the lens quality).
 
CGrieves said:
Yes, but in my experience there's no point wrapping your head round 35mm equivalence while you're shooting. It's best just to understand what 18mm and 55mm look like on the camera you're using, not a hypothetical camera with a larger sensor that you're not!
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Im the opposite, i always convert to 35mm equivalant as that is what I know.

Eg 50mm is about natural view with your eyes, you need 28mm or ideally 24mm for landscapes etc.

It gets messy with difference cameras and formats otherwise.

In my case on my mircro 4/3 system i need a 25mm for the natural 50mm effective and ideally a 12mm lens or less for my landscape shots.
 
Get a ruler out and measure the lens from the front element to the back, that is basically the focal length for tele lenses. Seriously that is all focal length means. For lenses wider than the height from the mount to the sensor then special retrofocus lens designs are needed and the lenses get bigger again, otherwise a 300mm f/4 lens is effectively a 300mm long tube with a glass 300/4=75mm diameter glass front element.
 
D.P. said:
Get a ruler out and measure the lens from the front element to the back, that is basically the focal length for tele lenses. Seriously that is all focal length means. For lenses wider than the height from the mount to the sensor then special retrofocus lens designs are needed and the lenses get bigger again, otherwise a 300mm f/4 lens is effectively a 300mm long tube with a glass 300/4=75mm diameter glass front element.
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Just read the writing on the lens :P
 
D.P. said:
Get a ruler out and measure the lens from the front element to the back, that is basically the focal length for tele lenses. Seriously that is all focal length means. For lenses wider than the height from the mount to the sensor then special retrofocus lens designs are needed and the lenses get bigger again, otherwise a 300mm f/4 lens is effectively a 300mm long tube with a glass 300/4=75mm diameter glass front element.
Click to expand...

That doesnt work for every lens.

Take the olympus 75mm f1.8

69.5mm length (acceptable as there is then the distance from the lens mount to the sensor which then should make it 75mm.

However front glass element is 58mm ???? It should only need to be 41.6mm by your theory?
 
Thank you very much for all the replies. The main reason I am asking, apart from wanting a better understanding, is that I am looking for a new lease for landscape and astrophotography (Milky way). These are the lenses I have been looking at:

  • Sigma 10-20mm f3.5 EX DC HSM Lens for Nikon Digital SLR Cameras with APS-C Sensors
  • SAMYANG AE 14mm f / 2.8 ED IF UMC wide angle Lens - for Nikon (Manual focus)
  • Samyang 35 mm F1.4 Manual Focus Lens for Nikon AE (Manual focus)

Also would they be worth it over the NIKKOR 18–55mm f/3.5–5.6G VR II I already have?

My understanding so far is that for astrophotography a lower F value will capture more light, which will enable you to decrease the shutter time or let you capture more light for longer exposure times. Also if the focal length is lower then you increase the shutter time and still avoiding star trails. Wide angle length to capture more of the sky.

Is it the focal length that makes a lens a wide angle lens? Thats what it says around the web. Or is it other things that also makes a sense wider.

I know that the questions are a bit stupid, but I just want to do more than just point and click....I want a better understanding what I do and what I am using.
 
the smaller the focal length the wider the field of view of the lens so you are correct.

The sigma been a APS lens would give you the widest angle of view. It is effective a 15-30mm lens in 35mm format.

Your Nikkor is a 27-82.5mm lens.

The sigma would give you 100 degress of FOV. Your current lens is only giving you 67 degrees so thats a massive difference.

http://www.bobatkins.com/photography/technical/field_of_view.html
 
Really for astrophotography you want a lens with minimal coma, which is the tendency to distort the shape of stars towards the edge of the frame. Primes tend to perform better in terms of coma (it's a not a rule though). Also Samyang lenses tend to be pretty good for this, they're cheap, and manual focus isn't a disadvantage for astro.

As mentioned above to avoid star trails you'll need all the aperture you can get, and combine it with high ISOs. The 5300 is a good body for high ISO without horrible noise.

Ideally you'll want a high quality tripod and a remote shutter release too.

If you need a wider view of the milky way, just walk further away from it (joke).
 
Greebo said:
Before you know it, you will be doing this

vtW5gHU.jpg

from memory that was around 30 seconds at f/2.8, ISO 1250
Click to expand...

That is what I want to be doing! Very nice photo!

That link you sent Greebo, was very useful, so thank you.



CGrieves said:
Really for astrophotography you want a lens with minimal coma, which is the tendency to distort the shape of stars towards the edge of the frame. Primes tend to perform better in terms of coma (it's a not a rule though). Also Samyang lenses tend to be pretty good for this, they're cheap, and manual focus isn't a disadvantage for astro.

As mentioned above to avoid star trails you'll need all the aperture you can get, and combine it with high ISOs. The 5300 is a good body for high ISO without horrible noise.

Ideally you'll want a high quality tripod and a remote shutter release too.

If you need a wider view of the milky way, just walk further away from it (joke).
Click to expand...

Thats why I had the Samyang lenses on the list. Reviews say that they are very good in regards to coma. Did not realise that primes was one of the reasons.

I have a good tripod and I am about to order a shutter release.

I am a bit worried about how to distinguish between noise to stars at high ISOs :o
 
Depends on the camera as to how much ISO you can get away with. On mine its 3,200 although I try not to go above 1600 ideally.

I find that's the best compromise and I know noise wont be an issue. Makes exposures around the 30s mark for me which is short enough that I don't get any star tracking (unless that is what I am after ;))

I suspect the SAMYANG AE 14mm f / 2.8 ED will be the best choice on your list. Like you say AF is irrelevant for what you are photographing and its a prime lens and decent(ish) speed at f2.8.
 
Greebo said:
That doesnt work for every lens.

Take the olympus 75mm f1.8

69.5mm length (acceptable as there is then the distance from the lens mount to the sensor which then should make it 75mm.

However front glass element is 58mm ???? It should only need to be 41.6mm by your theory?
Click to expand...

Yes, the focal length is the length from the middle of the front element to the sensor plane, bits of lens hood or whatever can extend beyond. On Mirrorless the mount to sensor distance is about is about 20mm, on DSLR about 40-45mm.

The front element aperture will vary a little, typically the front thread will be rounded up to some useful number like 77mm on FF lenses, 67mmmm on Nikon DX lenses with diameters like 52mm being common for small primes, 82mm for bigger lenses, 95mm and also the need for autofocus and IS?VR can make the lens fatter around the waist but the diameter is kept near constant.

Secondly, one way to get higher image quality into the image corners is to design the ens with a bigger image circle in mind and a larger front element which is used effectively stopped down a little. On lenses with smaller front element requirements it can simply be cheaper to use a front element that is larger than need be which gets manufactured in higher quantities, e.g. the 75mm f/1.8 might only need a 42mm lens element but a if they have a load of high quality 50mm elements at a suitable price then it is cheaper to use the, than manufacture bespoke glass.


And then yes, the rare exceptions like the new canon and Nikon DF/PF lenses that can be made much smaller, and other lenses that are way larger than need be to improve image quality. The 24-70mm is a good example, 70/2.8 = 25mm but the front element is much larger, in part to handle the 24mm end which is more complex with a wider angle of view.


Anyway, my point is the focal length is simply a physical measurement of the properties of the glass elements, so it is irrelevant what size sensor it is on because the glass doesn't change.
 
The Samyang 14mm f/2.8 is well regarded for astro work on a budget. It s not eprfect, but the perfect astro lenses are big money. Some people also prefer the f/1.4 primes. Sigma has just announced a 20mm f/1.4 lens which could be amazing for astro work if the coma is well controlled since you are 2 stops faster than the f/2.8 Samyang.
 
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