Crop sensor and impact on lens focal length

[PJM004]

I use a 7D with crop sensor and am considering some additional lenses. Does a crop sensor camara only 'crop' the image? If so, I would think it not completely accurate to give a 50mm lens the attributes of an 85mm lens. I am of the understanding that a longer focal length begins to compress the image. Compressing an image is different than cropping an image - I think. I don't hear this talked about. Does someone know? It may influence the choice of lens that I look at.

Another example, would a full frame 200mm lens picture look the same as a 135mm lens picture on a crop camera?

 

[o hey tyler]

I use a 7D with crop sensor and am considering some additional lenses. Does a crop sensor camara only 'crop' the image? If so, I would think it not completely accurate to give a 50mm lens the attributes of an 85mm lens. I am of the understanding that a longer focal length begins to compress the image. Compressing an image is different than cropping an image - I think. I don't hear this talked about. Does someone know? It may influence the choice of lens that I look at.

Another example, would a full frame 200mm lens picture look the same as a 135mm lens picture on a crop camera?

Yes, the crop sensor is only displaying a middle portion of what a full frame sensor would be. Therefore the lens characteristics are not changed. There's a very big difference between a 50mm on a crop frame camera and an 85mm on a full frame.

 

[orb9220]

As pointed out it doesn't affect the focal length so much.
As change the FoV (Field of View) that is seen thru the viewfinder & what the sensor records.
.

 

[480sparky]

Click here.

 

[Gavjenks]

Yes, it is cropping out part of the optical image. However, it also does so with many more pixels, usually (not always! Some weird full frames actually have 2x as many pixels and maintain the same density).

Due to the higher pixel density, if your lens is "sharp" enough, you should be able to resolve more on a 50mm on a crop sensor than from a 50mm on a full frame with the same image cropped out of the middle of it.

So it is not actually equivalent to just "merely croppping" as if in photoshop. It is not as bad as that.



It is not fully equivalent, though. If you can afford the longer equivalent focal length for full format, an 85mm FF shot will pretty much always be better than a 50mm crop sensor shot of the same scene.

The "if you can afford it" part of that sentence is crucial, though. The jump from 500mm to 800mm equivalent quality glass could be like, 3-5x more expensive, unlike the jump from 50mm to 85mm, which is basically free. Crop sensors thus shine at long focal lengths, where you can get almost-equivalent IQ for vastly less lens money.

 

[480sparky]

Yes, it is cropping out part of the optical image. However, it also does so with many more pixels, usually (not always! Some weird full frames actually have 2x as many pixels and maintain the same density).

Due to the higher pixel density, if your lens is "sharp" enough, you should be able to resolve more on a 50mm on a crop sensor than from a 50mm on a full frame with the same image cropped out of the middle of it.

So it is not actually equivalent to just "merely croppping" as if in photoshop. It is not as bad as that.



It is not fully equivalent, though. If you can afford the longer equivalent focal length for full format, an 85mm FF shot will pretty much always be better than a 50mm crop sensor shot of the same scene.

The "if you can afford it" part of that sentence is crucial, though. The jump from 500mm to 800mm equivalent quality glass could be like, 3-5x more expensive, unlike the jump from 50mm to 85mm, which is basically free. Crop sensors thus shine at long focal lengths, where you can get almost-equivalent IQ for vastly less lens money.

The only consistency about crop sensors is the 'FOV magnifier' to an equivelant full-frame lens, and even that varies by manufacturer. "Crop sensor" NEVER means "X fewer pixels compared to full-frame".

 

[PJM004]

Thanks for the feedback. Just to be clear, my compression comment did not have anything to do with the number of pixels. Has to do with the visual impact of bringing distant items closer together when using longer focal length compared to shorter. So if in your photo, you want the impact of the visual compression you CANNOT conclude that you will get the impact with a shorter lens on a crop camera that has some equivalent length of full frame.

This makes sense to me. It seems that there is a lot of mis-leading info that would lead to confusion for those with less experience.

Thanks again.

 

[Helen B]

Thanks for the feedback. Just to be clear, my compression comment did not have anything to do with the number of pixels. Has to do with the visual impact of bringing distant items closer together when using longer focal length compared to shorter. So if in your photo, you want the impact of the visual compression you CANNOT conclude that you will get the impact with a shorter lens on a crop camera that has some equivalent length of full frame.

This makes sense to me. It seems that there is a lot of mis-leading info that would lead to confusion for those with less experience.

Thanks again.

I think that's misleading. If the pictures are taken from the same place, with the same angle of view, and displayed at the same size the perspective will appear the same - the compression effect will be the same. It's fairly simple geometrical similarity.

 

[PJM004]

I had started to think about the last response before it came in. But I am still not sure now on the geometry. While the FOV may be the 'same' at the camera positions, a lens of higher magnification (longer focal length) should be narrowing the FOV as items increase in distance from the camera (hence the compression). Earlier comments in the thread suggested the the two compared results would be very different. Any more thought on this?

 

[kundalini]

If you've never shot film or full frame sensor before, the point is moot. Regardless of the sensor size, the focal length stamped on the lens is accurate. 50mm is 50mm, 300mm is 300mm, and so on.

 

[Helen B]

I had started to think about the last response before it came in. But I am still not sure now on the geometry. While the FOV may be the 'same' at the camera positions, a lens of higher magnification (longer focal length) should be narrowing the FOV as items increase in distance from the camera (hence the compression). Earlier comments in the thread suggested the the two compared results would be very different. Any more thought on this?

The focal length of the lens has no effect on the relative sizes of the images of objects at different distances. Those relative sizes are set by the location of the camera (specifically the lens' entrance pupil). If the overall FoV of two sensor/lens combinations is the same, how can there be some magical local change in FoV that is caused by the focal length of the lens (if the lenses are rectilinear)?

 

[coppertop24]

Ok, so Ive only been doing this for a few weeks, but the way I have understood all of this from my research is that the focal length between the two sensors using the same lens wont change, only the amount of the image that is captured. This is where folks get the "crop factor". As mentioned earlier, if you are using a 50mm on a FF, crop out the middle at the size of an APS-c sensor and you would have the FOV equivalent to an 80mm lens. BUT, the biggest difference comes in noise and pixel density when enlarging images. If you have a FF at 18MP and a crop sensor at 18MP, the APS-c will be sharper than the FF at native size due to the higher pixel density. Now, this isnt considering noise at high ISO, because FF do better at higher ISO than APS-C (crops). But, if you take the APS-C image and enlarge it to the same size as a native FF image, the FF will be sharper. Its kind of like the difference between a 1080p HD phone screen and a 1080p HD 60' tv. Both look good at native sizes (phone will look terrific due to more pixels in smaller area), but blow the phone up to 60' without increasing pixel count and it will look horrendous. (kind of extreme, I know)

I know there is much more to this and Ive over simplified and possibly butchered it, but Ive had a bottle of wine, so forgive me a little. BTW, been enjoying the forums and the recommendations coming from everyone. Its interesting to see how different everyone views what they want to see in their pictures and equipment.

And I would stop thinking of compression, especially in relation to FOV and distance from the sensor. If you narrow the FOV while increasing the distance of an object from the sensor, you could theoretically maintain the same image as if you were closer with a wider FOV. "Compression" wont affect the image so much as the amount of light the lens is capable of gathering at longer focal lengths which will also determine ISO and shutter speed. If you are forcing higher ISO due to a slow lens, you run the risk of more noise depending on the body.

 

[Gavjenks]

If you have a FF at 18MP and a crop sensor at 18MP, the APS-c will be sharper than the FF at native size due to the higher pixel density.

Yes, as long as:

1) Your lens is actually sharp enough to OPTICALLY resolve precisely enough for the smaller pixels to have any advantage in the first place, and
2) The crop sensor does indeed have higher pixel density. In your example, it does, but there are some FF's out there that actually have the same pixel density as equivalent crop sensors by the same company (example: Nikon full frame D800 has HIGHER pixel density than the crop sensor Nikon 3100). In such a situation, there would be absolutely no advantage to the crop sensor over the FF as far as sensors go, except for price, because you could literally just crop out the smaller area in photoshop and have the exact same image, more or less pixel for pixel, as a crop sensor with the same lens.

 

[coppertop24]

If you have a FF at 18MP and a crop sensor at 18MP, the APS-c will be sharper than the FF at native size due to the higher pixel density.

Yes, as long as:

In your example, it does, but there are some FF's out there that actually have the same pixel density as equivalent crop sensors by the same company (example: Nikon full frame D800 has HIGHER pixel density than the crop sensor Nikon 3100). In such a situation, there would be absolutely no advantage to the crop sensor over the FF as far as sensors go, except for price, because you could literally just crop out the smaller area in photoshop and have the exact same image, more or less pixel for pixel, as a crop sensor with the same lens.

Well, yeah. The D800 is a 36MP sensor compared to a 14MP in the 3100. I used 18MP for both to simplify my example. And my examples were under all conditions equal. I guess I just figured that was implied. But thanks for interjecting, i guess. Relax, Im not trying to say crop is better than FF, I know better.

 

[Helen B]

And I would stop thinking of compression, especially in relation to FOV and distance from the sensor. If you narrow the FOV while increasing the distance of an object from the sensor, you could theoretically maintain the same image as if you were closer with a wider FOV.

For most three-dimensional subjects that isn't really true in theory or in practice, except for those that are very far away. The effect of what the OP is calling compression is one of the main choices you make as a photographer, by choosing distance and angle of view. This choice of distance and angle of view is very commonly applied in portraiture, for example, when greater distances and narrower angles of view are usually preferred over short distances and wide AoV (for similar FoV) because the images are quite different.

The idea that this is a property inherent in focal length across formats (eg that a 14 mm lens will produce 'distorted' head shots no matter what format it is used on, or the converse mentioned in the original post) is a fallacy that pops up fairly often.