Digital Prime Lenses?

[MBasile]

So I know with the smaller frame DSLR's the focal length is multiplied (~1.5x for mine). Knowing this, wouldn't getting a 30mm (well, 33 would be optimal) prime lens make more sense than a 50mm? 50mm on film gave the perspective of the human eye, and unless I am missing something, it seems to me that 30mm on an APS-C sensor would be the digital version, correct?

 

[Josh66]

More or less...

But you would also get the perspective changes and distortion that you get as you go wider. It wouldn't look exactly the same as a 50 does on a 35mm camera.

 

[MBasile]

Thanks for the input... I'm still trying to wrap my mind around the how the lenses act in the digital world.

 

[musicaleCA]

Yeah, not exactly the same, but pretty darn close.

On an APS-C sensor, the crop factor is 1.6.

 

[MBasile]

Yeah, not exactly the same, but pretty darn close.

On an APS-C sensor, the crop factor is 1.6.

Sony's has always been listed as 1.5, I think Canon's is around 1.6, but don't quote me on the Canon number

So basically, it'd give me a similar field of view as a 50mm on film, but some wide-angle-like distortion?

 

[musicaleCA]

Maybe a little; but that distortion is worst at the edges anyway, and those are getting cropped-out.

My bad; yeah, Sony's are 1.5. Silly brain.

 

[Josh66]

You would probably notice the difference on, say - a close-up of someone's face.

Other than that, I wouldn't spend too much time worrying about it.

I also wouldn't worry about crop factors too much. If what you have now isn't wide enough, get something wider. If it's not long enough, get something longer.

Does it really matter if it's the same as it was on 35mm?

 

[Garbz]

Wait Wait Wait. Take your ultra wide angle and crop it in the centre. All that distortion appears at the edges of the frame.

What you get when you mount a 35mm on a APS-C type camera is the equivalent field of view as a 50mm, meaning all perspective distortion etc is as it would appear mounting a 50mm on film. Think about it, how else could the 3-10mm lenses on point and shoots get a usable picture.

The only thing that changes is the depth of field, because now you have a 35mm lens with the middle croped out so your effective camera to subject to background ratio changes. Thus the only difference is that a 35mm f/1.4 on APS-C would have less depth of field as a 50mm f/1.4 on film. Everything else is pretty much the same.

 

[Josh66]

What would you call this?

I guess it's more the distance to the subject than the actual focal length that causes it - but shorter focal lengths will require you to move closer to the subject get the same composition as the longer one.

Or the effect seen here. (half way down the page)


Maybe 'perspective distortion' is the wrong term, but something is going on and I don't really see how cropping would fix it.

 

[KmH]

You shoot from farther away to minimize the distortion and then crop.
Some of those short FL model images, the lens was almost touching her nose.

 

[itznfb]

Maybe a little; but that distortion is worst at the edges anyway, and those are getting cropped-out.

My bad; yeah, Sony's are 1.5. Silly brain.

nikon is also 1.5

 

[Josh66]

You shoot from farther away to minimize the distortion and then crop.

That doesn't seem like a very good solution.

I wouldn't use a lens that required me to crop just to get a decent image...

I'm not sure that that would completely fix it anyway. Not to mention that you would be throwing away the majority of the frame...

Don't get me wrong - I'm not saying wide angle lenses are crap, just that they shouldn't be used for portraits. Not head & shoulders shots anyway. It would probably be fine for a full body shot.

Some of those short FL model images, the lens was almost touching her nose.

That's the point. You have to be so close to fill the frame that you end up with crap results.

If the only alternative (other than using a more appropriate lens) is to stand far away and crop most of the picture out...well, that's just not really a good option.

 

[Garbz]

Jeep, perspective distortion IS the correct term, but I think it's the understanding of how it is applied to cropped cameras, which literally can be thought of as cropped. The distortion is a function of angle of view of the image, as well as subject to camera distance.

However if you had to resize the subject to fit in an area 1.5x smaller for any given focal length with the intent of cropping as an APS-C sensor would do, you are changing the the subject to camera distance, and changing the resulting perspective.

Thus when shooting with a 30mm on an APS-C camera cropping to give a 50mm field of view, you would need to stand an almost equal distance from the subject as with a 50mm.

Thus same angle of view, same distance to subject, same perspective distortion. You could treat a 30mm on an APS-C almost identically to a 50mm on full frame.

 

[patrickt]

"Thanks for the input... I'm still trying to wrap my mind around the how the lenses act in the digital world. "

They work exactly the same as they did in the film world. A 50mm was the "normal" lens only in the 35mm film camera. With other film formats, other lens lengths were "normal". Same thing with digital.

 

[Dwig]

What would you call this?

I guess it's more the distance to the subject than the actual focal length that causes it - but shorter focal lengths will require you to move closer to the subject get the same composition as the longer one....

You're a bit confused.

One, its only the distance, not mostly the distance, that created the effect seen in the pictures to which you linked.

Two, what we are talking about here is not "distortion", its more properly "perspective alteration". You see a perspective that is perfectly normal but your brain is disturbed because it is missing the distance cues when viewing the image. The result is the perspective is different from what your brain expects given the viewing distance to the print/screen.

Three, you wouldn't move any closer when the shorter focal length lens is used on a proportionally smaller format sensor. A 50mm lens on a 24x36mm sensor or film will yield the same field of view at the same distance as a 33mm lens on a digital sensor with a 1.5x "crop factor".

To the OP's question. No, it doesn't make more sense to get a 30mm lens than getting a 50mm lens. It doesn't make less sense either. The question is why are you getting the lens. One reason that fast 50's are popular on crop sensor DSLRs is that they make great portrait lenses for loosly framed 3/4's portraits. True, the longer 85's are generally better choices for tight face shots. If, and only if, what you are looking for is a "normal" lens (normal = focal length equal to the diagonal of the image) then a 28-30mm lens is what you would look for on the common DSLR. The common "standard" 50 is somewhat longer than a normal for 24x36 format; a 40-42mm lens would be more proper.