Lenses and focal lengths on a APS-C camera

crazyswede · 14 Oct 2015 at 17:48

I am now thinking about getting a cheaper lens to mostly use it for astrophotography and also try to use it for now for landscapes. The Samyang (also known as Rokinon) seem to be good for this. I don't know which one will be better of the two (Well I think I know):

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)

One has a good focal length but a higher aperture number and the other one is higher focal length but lower aperture. I think that as I have a APS-C camera I should go for the 14mm f / 2.8. It is also £70 cheaper.

How do you guys focus for these milky way shots?

Greebo · 14 Oct 2015 at 18:40

I just focus to infinity and i think the 14mm will be better as the other one will be 50mm on your body and not wide enough.

And i find f2.8 fine for star shots

Vertigo · 14 Oct 2015 at 18:45

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.

Err, what? This makes no sense whatsoever.

There are plenty of lenses where this is nowhere near the case, as well as many different lenses of the same focal length which differ massively in both length and diameter.

A simple example - my Canon 50 f/1.4 is massively shorter than my Sigma 35 f/1.4.

Greebo · 14 Oct 2015 at 19:36

Vertigo1 said:
Err, what? This makes no sense whatsoever.

There are plenty of lenses where this is nowhere near the case, as well as many different lenses of the same focal length which differ massively in both length and diameter.

A simple example - my Canon 50 f/1.4 is massively shorter than my Sigma 35 f/1.4.

I did say his theory didnt work

D.P. · 14 Oct 2015 at 19:56

Vertigo1 said:
Err, what? This makes no sense whatsoever.

There are plenty of lenses where this is nowhere near the case, as well as many different lenses of the same focal length which differ massively in both length and diameter.

A simple example - my Canon 50 f/1.4 is massively shorter than my Sigma 35 f/1.4.

Someone didn't read what I said about focal lengths shorter than the flange (mount to sensor) height. The 35mm ART is a retro-focus design and is irreverent to the point I was making.

D.P. · 14 Oct 2015 at 20:01

Greebo said:
I did say his theory didnt work

It is not my theory, it is just a fact, and it does work, because it is a fact.
You are getting way too worked up about the details if you think a counter example disproves the general point that I was making - focal length by definition is the length form which light focuses on a surface by a lens element.

https://en.wikipedia.org/wiki/Focal_length

For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens.

Greebo · 14 Oct 2015 at 20:02

SUrely that is only 100% true in the case of one single lens and gets muddied with lense consisting of multiple pieces of glass?

D.P. · 14 Oct 2015 at 20:20

Greebo said:
SUrely that is only 100% true in the case of one single lens and gets muddied with lense consisting of multiple pieces of glass?

In general not. My 70-200mm is 200mm long, my 300mm ED is 300mm long etc.

lenses can be made larger with additional elements, soemthign whcih is necessary for retro-focus designs. And with special element like Nikon's Phase Fresnel design somewhat shorter, but that is completely missing the point that the focal length stated on the lens is in essence a physical measurement of , you know, the focal length of the glass lens element.

Greebo · 14 Oct 2015 at 20:31

it is but doesnt really help the Op with his questions regarding focal length on his APS-C body though?

I have a phone camera with a focal length of probably 3mm. That means nothing but makes sense to me when I convert it back to 35mm equivalent.

D.P. · 14 Oct 2015 at 20:44

Greebo said:
it is but doesnt really help the Op with his questions regarding focal length on his APS-C body though?

I have a phone camera with a focal length of probably 3mm. That means nothing but makes sense to me when I convert it back to 35mm equivalent.

Actually, my point was very helpful to the OP until people started trying to poke holes in a basic fact and showing counter examples that I explicitly excluded.

The OP was confused at focal lengths and how the sensor comes to play, how lenses designed for APS-C and FF are comparable. The simple fact is Focal length is just a basic physical measurement of the lens so the sensor is irreverent, as is the lens being designed for APS-C or FF.

The OP doesn't have a FF camera, is not used to FF 35mm film cameras, and so the entire comparison is fairly useless. He simply needs to know what different focal lengths look like, and know that it is irrelevant if the lens was designed for crop or not.

Here is a useful tool.
http://www.tamron-usa.com/lenses/learning_center/tools/focal-length-comparison.php

Greebo · 14 Oct 2015 at 20:50

Im okay with what you said except for the last bit.

In the op's case he is looking at lenses designed for the APS-c and others not designed for that system. Telling the op that a 50mm lens is a 50mm lens whatever the body its connected to, albeit technically correct, is just going to confuse him.

I could put 50mm FF lens on my micro 4/3 body with an adaptor but it would cease to give the same FOV as it did on the FF body.

Im not been argumentative and and yes I know focal length + focal length whatever the body its attached to but I alwasy prefer to cross-reference back to equivalent 35mm which is afterall what 95% of all review sites do as well.

crazyswede · 14 Oct 2015 at 22:40

You have all been helpful and i don't mind you going off on a tangent

I forgot that there is this option as well:

Samyang 24 mm F1.4 Manual Focus Lens for Nikon AE

But it is £130 more. I think I am still better off starting off on my quest with the following (Am I wrong?):

SAMYANG AE 14mm f / 2.8 ED IF UMC wide angle Lens - for Nikon

Greebo · 14 Oct 2015 at 22:53

Personally id got for the 14mm. I prefer wider angles and f2.8 is fine.

If you love and want better latter then you can upgrade to a 10-16m prime with f2 or less

squerble · 14 Oct 2015 at 23:01

Don't worry too much about all the intrinsics. Just know that your crop factor is 1.5, so multiply whatever focal length you're at by 1.5 to get the "equivalent" full-frame view.

If you are looking at any famous photographers' pictures, it's likely they're shot on a full-frame camera, so if you want an equivalent FOV (field of view) then you need to divide by 1.5 to know what focal length to aim for on your camera.

Eg. Famous photograph is shot at 50mm on a full-frame camera, so you'd need a ~33mm on your crop camera to get the same field of view.

crazyswede · 14 Oct 2015 at 23:27

I am going to go with the 14mm f / 2.8. In the future, if I get good and enjoy it, I will invest in a better lens or camera or both

It will most likely be a lens first, but we will see. I will post some images if I succeed.

What do you think about light pollution filters if I want to try it in areas with more light?

D.P. · 15 Oct 2015 at 22:25

Greebo said:
Im okay with what you said except for the last bit.

In the op's case he is looking at lenses designed for the APS-c and others not designed for that system. Telling the op that a 50mm lens is a 50mm lens whatever the body its connected to, albeit technically correct, is just going to confuse him.

I could put 50mm FF lens on my micro 4/3 body with an adaptor but it would cease to give the same FOV as it did on the FF body.

Im not been argumentative and and yes I know focal length + focal length whatever the body its attached to but I alwasy prefer to cross-reference back to equivalent 35mm which is afterall what 95% of all review sites do as well.

The OP only has an APS-C camera so everything else is irreverent and highly confusing, including your statement that "he is looking at lenses designed for the APS-C." If the OP buys the Nikon 35mm f/1.8 DX or Nikon 35mm f/1.8 FX lens he will get exactly the same field of view, and that is what matters.

Who cares what 50mm looks like on m4/3 cameras, there is far more needless complexity thinking like that. For example, on m43 the sensors have a different aspect ratio so that single a multiplication by a crop factor is an incorrect way of understanding the horizontal and vertical field of view.

The field of view will change if the OP buys a FF camera, and that will require getting used to if and when that happens, no need to worry about hypothetical scenarios like a the crop factor of Phase One medium format back vs a 5"x4" Large format bellows camera or a 1" Nikon CX camera.

CGrieves · 16 Oct 2015 at 10:23

crazyswede said:
I am going to go with the 14mm f / 2.8. In the future, if I get good and enjoy it, I will invest in a better lens or camera or both It will most likely be a lens first

Unless you need autofocus the 14mm pretty close to as good as it gets- top notch resolution even wide open. It's definitely not a beginner throw-away lens anyway!

Focusing is easy on an ultra wide angle anyway! I use the 12mm f2 all the time.

What do you think about light pollution filters if I want to try it in areas with more light?

You won't be able to fit conventional screw-on filters due to the size of the front element. You'd need a glass filter and holder, but broadband filters in that format aren't likely to be cheap! Easier to find to darker skies....

crazyswede · 18 Oct 2015 at 23:27

CGrieves said:
Unless you need autofocus the 14mm pretty close to as good as it gets- top notch resolution even wide open. It's definitely not a beginner throw-away lens anyway!

Focusing is easy on an ultra wide angle anyway! I use the 12mm f2 all the time.

You won't be able to fit conventional screw-on filters due to the size of the front element. You'd need a glass filter and holder, but broadband filters in that format aren't likely to be cheap! Easier to find to darker skies....

I did not mean that it was a throw away lens

As it is cheaper it is a step towards other lenses but I will continue to use it.

It is manual focus, but he camera will indicate in the view finder if it is in focus. With my camera you have to set the aperture to F22 and then control the aperture in the camera settings.

Of course, it has just been cloudy every night, so no star photos. Here is one of the first photo with the lens whilst playing around with it:

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