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I have in fact read the other threads, and I am still not clear on this topic.

For any given lens, say a 400mm EF or FX telephoto, I am shooting a subject in low light, for arguments sake lets say I am shooting an eye chart (the kind you read in the military to check your vision).

The distance of the subject is such that no manner of focus or camera settings or photoshop manipulation will make the bottom line of the chart readable. The 2nd and 3rd lines to the bottom are almost readable when shot with the 400mm lens on a full frame body.

Will using a crop sensor body with the 400mm lens (having the effective focal length of 600mm) actually allow me to have better detail in my final image, and make those unreadable lines appear readable?

For arguments sake the camera bodies and sensors are identical in every regard except the crop factor, and forgetting arguments about final photo print size, pixel size, depth of field, etc., and only worrying about the final ability of the camera system to resolve detail in written words at extreme distances...

Logic would dictate that the answer is no; otherwise why wouldn't they make 5x or 10x crop sensor cameras so people could take advantage of insane effective focal lengths from their lenses? However I am seeing both answers when I google this, and a lot of confusing explanations.

Thanks!

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    When you say the sensors are identical do you mean the same number of different sized pixels or a different number of the same sized pixels? It affects the answer to the question. So does whether the absolute resolution of the lens is higher or lower than the absolute resolution of the sensor. – Michael C Nov 23 '17 at 10:38
  • A crop sensor would give you more details, assuming it has the same or higher megapixel count as the full frame sensor. However it does so at the expense of light capturing ability, since the physical size of each pixel is now smaller. People don't bother making 5x or 10x crop sensors because the performance becomes dysmal despite the apparent benefit in focal length - although the Nikon 1 is a 2.7x crop and can offer a very compact super telephoto set up (if it isn't getting killed off by Nikon, at least). – Semaphore Nov 23 '17 at 12:16
  • They do make "super zoom" cameras with small sensors allowing moderate telephoto lenses to have very small angle of views. These are mostly in the point-and-shoot category. The image quality does tend to suffer in comparison to APS-C because of the small sensor (the sensor wells tend to be smaller and packed in tighter, which exaggerates limitations in the lens' ability to focus). But plenty of people are happy with them. – David Rouse Nov 23 '17 at 13:38
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    @Semaphore Please don't answer in comments; instead, please write an answer. see Short answers as comments — please resist the urge – scottbb Nov 23 '17 at 13:38
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I am presenting you, the ultimate crop factor tool.

scissors

If it is an identical sensor, the only difference is that someone used one of these to fit the sensor in a smaller body.

The lens is the same, the distance is the same. It is only cropped.


You are starting with a wrong premise:

For argument's sake the camera bodies and sensors are identical in every regard except the crop factor, and forgetting arguments about final photo print size, pixel size.

As you can start seeing you cannot disregard pixel size, final print size, nor total megapixels or physical effects like diffraction.

Using the tool I mentioned: a pair of scissors, cropping the sensor is cropping the total Megapixels.

It would be exactly the same as printing a larger image on a larger paper, for example, a 16-inch paper and using the scissors to crop the final print to 10 inches. It is only the "perceived final framing" that counts.

Image of two pairs of rectangles representing sensors and papers


No. You do not build larger sensors only to play with focal length. You build larger sensors to fight the physical challenges that light faces on small sizes. Noise, sharpness, and diffraction.


Take a look at the image I posted on this answer: Do I use the crop factor in calculating aperture size and area?

having the effective focal length of 600mm

Crop factor is only a smaller sensor. It has NOTHING TO DO with focal length.

Here is a 50mm lens. It is 50mm regardless it is on a big frame camera, without a camera or a camera that has a cropped sensor.

A lens and a camera, a lens alone, and a camera chopped by a shark (cartoon)

A 400mmlens has an effective focal length of 400mm. Compare technical terms only if you are willing to understand the technical terms.


If we have a sensor cut out from the same silicon plate, a full frame sensor of 24Mpx, you would have more or less 27,777 pixels per mm2.

Cropped to an APS-C sized chip would be more or less a 9.4Mpx sensor, with the exact same 27,000 per mm2.

enter image description here

Now, comparing a 24Mpx full frame to a 24Mpx APS (Advanced Photo System) the second one has a different pixel density, so it is not the same sensor.

Greater pixel density is what gives this "tele" conversion.

If you are using a good lens, yes, you will have a final image with this "tele" conversion. If it is the objective, yes, for bird photography, wildlife and probably sports photography an APS camera is a good option if you have good light conditions.

But if you need to push the postprocessing of an image to squeeze detail from darker areas or low light conditions or sharper images overall, probably a full frame one is a better option.

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A crop-sensor camera having a focal length multiplier of 3/2, shooting with a 400mm focal length lens would be equivalent to shooting with a 600mm focal length lens and a full-frame sensor camera. That is really all you can determine about the given scenario without actually making the image (unless possibly you have much more knowledge than is typical of a photographer, e.g., an electronic engineer or physicist who designs these sensors.)Given that there would be no depth-of-field (dof) issue when photographing a plane object, the two scenarios should be equivalent. Generally, a crop sensor will have greater dof because a shorter focal length lens is required to capture a given image, but that wouldn't be an issue in the case of an image of an eye chart because the required dof is essentially nil. As the dof of the image you desire to capture increases, the advantage moves to the crop sensor camera and the 400mm focal length lens. In practice, however, it is only the extreme cases where this matters. Most images made by a $20,000 "professional" setup and a $5,000 "prosumer" setup are indistinguishble to most people. Someone who makes distant images professionally is going to opt for a full frame sensor camera and a longer focal length lens, and pay the much higher price for them, and control dof with aperture manipulation.

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Actually, the do make cameras with 4.6X and 5.6X crop sensors and, yes, the resolve insanely small details but the image quality is much lower because the sensor is so much smaller and cannot gather the same amount of light. This gives it higher noise and lower dynamic-range than larger sensors.

As an example, the Nikon P900 has a 5.6X crop and its lens is equivalent to a 24-2000mm lens. You can read my review of it here and see the sample gallery attached to the review. It can resolve very small details. Just look at the grass around the photo of the bird and take a look at how tightly it can frame the moon (although this is noisy since it's an ISO 400 shot).

A DSLR, APS-C or Full-Frame and most mirrorless, would have no issue with noise damaging detail at that ISO and nowadays often until ISO 3200.

Now onto your example, if you are shooting a wider scene and somewhere in it there is an eye-chart, then if you had a Full-Frame and an APS-C cameras with the same megapixel count, the cropped-sensor would resolve more details in the chart since its equivalent focal-length is longer and the chart would occupy a larger portion of the image (the outer part of the image being cropped out by the crop-factor). However, if you were to tightly frame the eye-chart, then you would have to back away to see it all with a cropped-sensor and you would end up with two images of exactly the same resolution which, excluding depth-of-field differences, would look pretty much identical.

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