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12-22-2013, 04:51 AM   #1
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Crop factor and lenses

Hi, I gathered as much and I hope its correct:
- the APS-C cameras have a crop factor around 1.5x.
- that means the effective focal length for old lenses is whatever is written on the lens x 1.5.

I'm still a tad confused though; technically 50mm prime is a 75mm prime. So if I want to shoot with a 50mm like the old film SLRs worked, I'd actually need to get a 35mm lens? But as far as my experience go larger field of view has more distortion, meaning 35mm x crop factor won't look the same as 50mm despite the fact that image should technically look the same size. Am I getting this right?

That would mean APS-c actually brings nothing good to the table at all, at least I can't see it. Seems like it does the same as a teleconverter would do (optically magnifies and crops). Except for telezooms, where you don't loose light and resolution to get greater detail.

Why didn't they bring the lens closer to the sensor so it wouldn't crop?

Are the lenses made for APS-C (that show a circle crop on a full frame) designated with crop factor being taken into account, meaning that 50mm for APS-c would technically not be the same as a normal 50mm?

Thanks

12-22-2013, 05:16 AM - 2 Likes   #2
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Hey there

QuoteOriginally posted by Ploki Quote
- that means the effective focal length for old lenses is whatever is written on the lens x 1.5.
No! Stop right there!
The focal length is a lens property and the medium (as in, sensor or film) does not affect it!
The only thing is that the crop sensor is effectively a smaller canvas, a smaller window, so the field of view is smaller. The edges get cut off, and effectively the lens appears to be less "wide."
Equivalence using that multiplier is only important if you use a film/FF camera and a crop camera, and you want the exact same field of view. Keep in mind that using different focal length lenses means that even if the field of view is the same, there will be different depth of field and distortion and space compression.

QuoteOriginally posted by Ploki Quote
35mm x crop factor won't look the same as 50mm despite the fact that image should technically look the same size.
You are right that they will not look the same. But you are wrong about the "technically" part. Equivalence means nothing, it doesnt exist, it is a made up concept. It merely helps a photographer visualize the field of view of a certain lens on a certain camera. If I tell you to think of a 25mm lens on the 645D or a 5mm lens on the Q7, you won't know what field of view you get. But if you use equivalence and multiply it, you will be able to compare it to the lens on your APS-C camera and easily visualize it. Of course, the field of view will be the only thing. Actual rendering could be quite different.

QuoteOriginally posted by Ploki Quote
That would mean APS-c actually brings nothing good to the table at all, at least I can't see it.
Well, some people would agree with you, and they want a FF (or bigger!). But crop sensor is much easier to produce (costs less), takes less space, allows for more compact camera designs and you can put more electronics in there (like in body image stabilization). And some people like that, since it usually has bigger pixel density, it can achieve higher apparent magnification or make lenses appear longer (more tele). So this can be useful for some macro and wildlife photography, as well as for hikers.


QuoteOriginally posted by Ploki Quote
Why didn't they bring the lens closer to the sensor so it wouldn't crop?
There is a whole science of optics. The distance between the lens and the sensor is precisely defined, and you cannot simply move that focal plane. The register distance is defined by the K-mount specifications. The other problem is the mirror, which needs some space between the lens and sensor. This is why only mirrorless cameras can have a lens mounted very close to the sensor. So if you want to change the register, you might as well change the whole mount. And putting the lens "closer" does not increase/decrease the image circle, but it would mess up the focusing and make everything less bright. Its a complicated issue

QuoteOriginally posted by Ploki Quote
Are the lenses made for APS-C (that show a circle crop on a full frame) designated with crop factor being taken into account, meaning that 50mm for APS-c would technically not be the same as a normal 50mm?
No, a 50mm lens from the 1970s made for film will show the exact same field of view as the DA 50mm, on the same camera. Focal length is focal length, serious photography brands do not write "equivalences" on the lens, but the actual physics. But if you mount a crop-only lens on a bigger sensor (like full frame or medium format), it might produce an image circle that is too small, and you will get dark edges. But again, do not buy an older/FA 50mm expecting it to behave differently than your new 50mm. The focal length is the same. And if you use it on the same medium/camera, the field of view will be the same.

Edit: Basically, the problem is that 35mm film became the standard and now all other mediums are judged by it. Crop sensor is not really "worse" but it is "different." But I am sure some will disagree and say that bigger is better. Ive edited the post instead of doubleposting, so there might be some more text since you read it. And thanks for looking at my 500px page

Last edited by Na Horuk; 12-22-2013 at 06:00 AM.
12-22-2013, 05:25 AM   #3
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Cool. clears some things up, i thought that the cropped lenses are being designated differently. That would be incredibly stupid though.

So focal length is the same but field of view is different. so aps-c really doesn't do anything good except that it crops the image.



Just noticed a lot of familiar places from your 500px page. :-)
12-22-2013, 06:54 AM   #4
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QuoteOriginally posted by Ploki Quote
Cool. so aps-c really doesn't do anything good except that it crops the image.
It doesn't crop anything. The term "Crop factor" is one of the big misnomers in photography, the term was originally coined to aid in explaining to 35mm film or early digital photographers how their current lenses would perform differently when using a camera with a smaller sensor than the standard 24X36 35mm dimension, hence it would require using a shorter millimeter or longer millimeter lens to achieve the same Field of View image than what they were currently used to getting with their 35mm slr or full 35mm dimension digital cameras lenses. One of the selling points at the time was one could carry smaller somewhat lighter lens than carrying a longer usually heavier lens. One of the differences between older slr lenses, digital full frame lenses and lenses made for smaller digital sensors is that the digital lenses made for the smaller sensors are designed to cast a smaller circle than a full frame lens to cover the sensor size, which also helps in the lenses being made smaller in the process.

What good it did was allow cameras and lenses to be made smaller, lighter and easier to carry much the same as what camera manufactures were doing back in the 60' & 70's into the 80's with film cameras & Slr's. Not withstanding the technology strides in materials used on cameras have aided the weight and size for full framed cameras too, still and all those cameras and lenses remain bigger and heavier than the smaller sensor cameras. Of course there is downsides to the smaller sensor cameras but today's technology has made strides to make those much better


Last edited by Oldbayrunner; 12-22-2013 at 08:37 AM.
12-22-2013, 08:51 AM   #5
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QuoteOriginally posted by Ploki Quote
That would mean APS-c actually brings nothing good to the table at all, at least I can't see it.
I agree, but it depends on who you talk to. Apparently some folks like small cameras, never understood it myself. In truth, macro shooters, wildlife photographers, and sports photographers all benefit from the "TC effect" of aps-c. Portrait and landscape shooters are better served by larger formats due to the increased depth of field control, and less diffraction.


QuoteOriginally posted by Oldbayrunner Quote
It doesn't crop anything.
Um... yeah, I'm pretty sure it does precisely that. And by 'pretty sure', I mean definitely. Put any 135 format lens on a film camera and you'll get one image, put the same lens on an APS-C camera without changing the camera's position, and you'll get a cropped version of your original scene. Now the exception might be when you're shooting a bird in flight for example. If the bird only fills a small portion of the frame on film, it will fill a slightly larger portion of the frame on a cropped sensor camera, but there will still be lots of sky that will need to be cropped in post.
12-22-2013, 09:24 AM   #6
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You might be getting confused but what both Na Horak and Oldbayrunner are saying is true. But to summarize in hopes that its clearer...

1. The old 50mm lens (say FA50) gives you the same perspective, DOF, etc. as the new 50mm lens (DA50)
2. The old 50mm lens (FA50) is designed for the larger full-frame sensor (35mm SLR or full-frame DSLR) so it has a larger image circle than the new 50mm lens (DA50)
3. The smaller image circle of the DA50 means it can only be used on cameras with a smaller APS-C sensor.
4. Given that you are standing in the same spot taking the same picture, the image taken with the DA50 on an APS-C sensor will be a cropped version of the image taken with the FA50 on a full-frame sensor - i.e. It will have a smaller field of view.

Hopes this helps.
12-22-2013, 12:30 PM   #7
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Okay, i think I get it. I also read somewhere that some lenses - especially cheaper ones - have poor resolution and distortion on edges, which is what essentially APS-c discards on lenses designed for full-frame.

I think I got it now. I also understand how small sensors with high pixel density can have such outrages focal length, although physical size is probably still something to consider

thanks all for answers
12-22-2013, 12:57 PM   #8
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QuoteOriginally posted by Ploki Quote
Okay, i think I get it. I also read somewhere that some lenses - especially cheaper ones - have poor resolution and distortion on edges, which is what essentially APS-c discards on lenses designed for full-frame.
Right, they call that the "sweet spot advantage." Some lenses which are lousy on film/full-frame because of poor edge performance are actually pretty good on APS-C, since the smaller sensor only sees the best part of the image. As far as I know, there's never been a lens that performs better at the edges than it does in the center, so this is always an advantage APS-C has over larger formats.

12-22-2013, 06:27 PM   #9
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Interesting thread. My question has to do with full-frame vs APS-C for the studio portrait photographer. Assume:

1. A former full-frame film studio photographer migrates to digital APS-C camera (assuming digital full frame is out of their price range), and
2. The photographer continues to take pictures with a 70mm to 90mm lens (for the usual reason: To achieve a flatter effect than the standard 50mm lens)

Is this photographer resigned to placing the camera 50% farther from his subjects than he did formerly in order to achieve the same field of view? If so, I should think this would be an issue for small studios, or for on-site portraits where space is an issue. Are there lenses, or lens adapters, that can be used to effectively squeeze the image on the sensor plane so that it fits on the sensor, so that the full frame field of view is simulated?

Thanks much in advance for your reply.
12-22-2013, 06:59 PM   #10
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QuoteOriginally posted by Kurt Euler Quote
Is this photographer resigned to placing the camera 50% farther from his subjects than he did formerly in order to achieve the same field of view?
You just stepped on a landmine.

You've probably heard the term "zoom with your feet" before. It's not a good expression, because it's simply not possible. Using a longer lens (or cropping) compresses the depth of the scene. If you move an APS-C camera back so that the subject fills the frame as much as it did with the same lens on a FF, the subject will have a flatter look to it, and the background will appear much closer. The pictures will be completely different.

As far as adapters to "squeeze the image" go, yes, they do exist. They're called telecompressors. The Metabones Speed Booster and the Lens Turbo are some examples of this.

Still, this is the reason why the whole "equivalent focal length" thing arose in the first place. If someone likes using a 70mm lens on full-frame, and they wanted to use an APS-C camera, they'd best switch to a 50mm lens or so for the smaller sensor.

Last edited by scratchpaddy; 12-22-2013 at 07:04 PM.
12-22-2013, 07:04 PM   #11
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QuoteOriginally posted by scratchpaddy Quote
As far as adapters to "squeeze the image" go, yes, they do exist. They're called telecompressors. The Metabones Speed Booster and the Lens Turbo are some examples of this.
But they are not for DSLR cameras. They are for MILCs.
12-22-2013, 07:07 PM   #12
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You got it, Kurt, although I would point out that you don't need to use a 70~90mm lens to get the kind of perspective compression that you are describing. A 55mm lens on APS-C gives the equivalent field of view, and subject compression that you would get from a classic 85mm portrait lens on a 135 film camera. The DA* 55 f/1.4 was designed to be a cropped sensor portrait lens.

And as far as using full frame lenses in the studio goes, I can give you an example. I have an FA 77 Limited. It's a great lens with beautiful rendition, but I don't like it as much on my K20d as I do on my film bodies. Why? It's the working distance. It forces me to stand too far away from my subject. There's a certain rapport with my subjects that I feel I lose when I'm far enough away to capture a 3/4 length shot. Luckily, I have a big enough studio where I can do that, but you're right, in a small space, you might not be able to get the shot at all. I guess I could get the DA* 55, but I end up using the DA* 50-135 because of the flexibility that it affords me.

There are "telecompressors", like the Metabones for certain smaller formats like micro 4/3rds that allow you to capture the entire image circle of a full frame lens as if the lens were being used on a body for which it was designed. So far though, no one had made such and adapter for a system like Pentax.

There is an issue, in that, with micro 4/3rds the flange distance is shorter than that for which the lens was originally designed, and there is no mirror taking up space, so the adapter can occupy the space that was originally occupied by the mirror box. Pentax on the other hand have never changed their flange distance, going all the way back to the m37 mount in 1952. So in order to make something like that for the Pentax system, in addition to compressing the image circle it would also have to compensate optically for the change in flange distance. That means more glass, more weight, more size and more expense. It's unlikely that you or I will ever see such and adapter.
12-22-2013, 08:10 PM   #13
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Thanks, all, for the quick comments --

Maxfied_photo, that DA* 55 f/1.4 seems like the best solution, although, at 55mm wouldn't the portrait photographer still have to put his camera farther back than in his full-frame days. Or, does the lens offer some built-in telecompression. (Product descriptions don't mention this.)

Anyway, at ~$800, it's beyond my reach right now.
12-22-2013, 08:33 PM   #14
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QuoteOriginally posted by Kurt Euler Quote
at 55mm wouldn't the portrait photographer still have to put his camera farther back than in his full-frame days.
You would stand in the same place with either a film camera and an 85mm lens, or a cropped sensor camera and a 55mm lens. That's how equivalency works; no magic, no compression. Put the 85 on the APS-C camera and now you have to stand back to frame the same shot. Put the digital 55 on the film body, and you've changed the shot. It's now wider, (i.e. more background), and the lens may not cover the entire image circle, although in this case I believe the DA* 55 does.

Once you get a solid grasp on that concept, then you can tackle DoF equivalency where it gets even more confusing.
12-22-2013, 09:01 PM   #15
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OK. Other option is to wait for the Pentax digital full frame. (It's availability is just around the corner you know. Has been for years! )
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