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I have a theoretical question. Consider two lenses with same f-ratio but one has a larger focal length than the other (say 200 mm vs 50mm). Which lens would give better image quality when shooting portraits in low light considering the same composition, same lighting conditions and same f-ratio? Camera shake, hand-holding, motion blur is neglected.

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    How do you define "image quality" ? – OnBreak. Apr 21 at 20:16
  • @Hueco From the SNR metric. – Suman Vajjala Apr 22 at 17:25
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    What has led you to believe that focal length, keeping all else constant, would have anything to do with image quality as judged by SNR? Sorry if this seems brash - it just seems to me like you've got a misconception somewhere. – OnBreak. Apr 22 at 17:37
  • @Hueco I do not think you are brash :) I thought, the longer focal length will collect more light than the shorter one for the same f-stop due to larger irix opening diameter. But, if the composition is kept the same then due to inverse square law the longer lens will see less light. So, I am not sure what would happen. Please note that this is just a technical question (and perhaps of no practical value) and the lens, sensor, etc are all considered ideal. – Suman Vajjala Apr 22 at 18:05
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    @SumanVajjala The larger entrance pupil is exactly offset by the higher magnification of the lens, which spreads what light it collects over a larger area. Or to look at it another way: It's angle of view is smaller, so it is gathering less light (due to the smaller area it is gathering from) and thus the opening must be larger to maintain the same amount of brightness projected onto the lens. – Michael C Apr 22 at 21:58
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"Image quality" commonly includes factors such as sharpness, resolution, distortions, aberrations, flaring, ghosting, etc. If all of these are identical between lenses, there would be no difference in image quality. In practice, it's unlikely you would have lenses that are exactly the same except for focal length.

There would also be no significant difference in low-light performance because you have posited that max aperture would be the same.

  • The F-stop system already accounts for light lost over distance. With the same F-number, lenses with longer focal lengths have physically larger apertures. (200/2 = 100) > (50/2 = 25).

  • There can be light transmission differences among lenses (T-stops). Focal length alone is insufficient to predict the T-stop of a lens.

There are two ways to achieve "same composition":

  • Move the 50mm lens closer to the subject. Some details will be relatively closer (noses) to the lens than others (ears). The closer details would be slightly easier to capture than the farther ones. However, it's not necessarily the case that the pores on people's noses are more important than those on their ears. Overall, such differences average out.

    You may also consider the perspective change caused by viewing the subject from different distances to be a factor in "image quality".

  • Use both lenses from the same position and crop the image taken with the 50mm lens. This will result in a loss of resolution. Grain or noise may also appear to be increased. Many would consider these to be reduced "image quality". However, the variance between center and edge would be reduced. Some people consider that improved "image quality".

Other factors:

  • Camera shake is more likely to affect a 200mm lens than a 50mm lens, even on tripod.
  • With great distances (miles/kilometers), the atmosphere itself will affect image quality. For distances typical of portraits, the effect should be insignificant.
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When imaging under feeble light conditions, need to use a large lens opening like f/2.8 or f/2 or f1.4 etc. Thus large f-number are a requisite, they capture more light. As alternatives, you can set the ISO high or use slow shutter speeds.

However, the key point that you are seeking: The f-number is a ratio that intertwines the focal length of the lens with its working iris diameter. Now a ratio is a dimensionless value. In other words, a lens set to f/4 affords the same exposure regardless. A 1000mm lens set to f/4 delivers the same exposure as a 25mm lens set to f/4. This is true for giant telescopic cameras as well. Thus there is no exposure advantage based on focal length differences.

However, portraiture is both an art and a science. As a general rule of thumb, facial distortion that results when working in too close is undesirable. A moderate telephoto fills this bill. Such a lash-up, 2X or 2.5X of normal focal length forces the photographer to step back and this deed greatly improves the resulting image of the human face. Let me aid, there are no rules in art, you are free to do your own thing.

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  • Hi.Thank you for the reply. As the 1000 mm f/4 lens will have more iris diameter than a 25mm f/4 lens, shouldn't it be collecting more light? Please tell me if I am missing some key information. – Suman Vajjala Apr 22 at 17:29
  • Yes - a larger working diameter lens collects more light. However, longer focal lengths yield larger images (higher magnfication) that spreads the light energy out over more surface area at the focal plane thus less light energy at any given point. Each doubling of the focal length results in a 4X light energy loss. The f-number system is a ratio that takes all this into account. Lenses set to the same f-number as another, deliver the same light energy (exposure) regardless of the dimensions of focal length or aperture diameter. – Alan Marcus Apr 22 at 18:29
  • Thanks Alan. I understand now. I also thought that to keep the composition same, one has to move back for the longer lens, thus, inverse square law comes into effect and cancels out the aperture advantage. Is this reasoning correct? – Suman Vajjala Apr 22 at 18:35
  • The camera lens acts like a slide or movie projector in that it projects an image of the outside world onto film or digital sensor. Consider – If a slide projector is moved, further away from a wall, the projected image increases in size. Double the distance, projector to wall and the image doubles in size. This is a 4X change in image surface area. Thus a 4X loss in image brilliance. You must open the iris 2 f-stops to compensate. Image was 4 X 4 now 8 X 8 a change of 4X in area. Same exact thing happens when you double the focal length i.e. 4X loss in image brightness, – Alan Marcus Apr 22 at 18:51
  • Thanks a lot Alan. I understand this now. – Suman Vajjala Apr 22 at 19:00
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The f-ratio defines the relation between the emitted light density on the scene and the arriving light density on the sensor.

The distance and focal length and crop factor and pixel size all are already compensated in that calculation.

That is important, for example, for placing off-camera flash. If you configure flash strength and distance for a certain ISO/f-ratio pairing, this ISO/f-ratio pairing on the flash will work with the same ISO/f-ratio settings on the camera regardless of where you move with the camera.

It also means you can hold a light meter before a spot of the subject that you want to meter in at nominal exposure, determine ISO/f-ratio for that spot of the subject, and then move with the camera to a different position and take the photograph with those settings.

If your camera has an exposure lock button, you can expose for a detail of the scene, then move to a different distance and take the shot.

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The shorter FL lens is most likely to generate images with better technical IQ for two reasons.

The first is because the lower magnification reduces the impact of motion (camera shake, subject motion, handholding)... but you said you want to ignore that.

That leaves the second reason... in order to record the same subject composition the lens must be nearer the subject. This means the details are presented physically larger to the lens, which requires less lens resolution in order to see/transmit them, and less sensor resolution to record them.

I.e. if you want the greatest detail/resolution then get as close as possible (e.g. macro w/ a 50mm on extension tubes)...

But the 200mm and 50mm images would be very different in other aesthetic considerations, such as how much BG is included and how blurred it is.

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  • You will need lenses of equally high resolution (lpmm) to capture equivalent details. As long as framing is the same, details will be about the same size relative to the frame. Consider how resolution is measured with a test chart. The chart is photographed to fill the frame with distance depending on the focal length of the lens. When photographing people, perspective will move some features (noses) relatively closer and some farther away (ears). But it's not necessarily the case that the pores on people's noses are more important than those on their ears. – xiota Apr 22 at 20:34
  • Yes, but you are asking much less of a shorter lens focusing on a target/subject at a shorter distance. The ultimate limit of lens resolution is diffraction; and the minimum size detail that can be resolved w/ a given amount of diffraction (F#) depends on their angular separation. If you take two diffraction limited lenses at the same F# they can resolve the same minimum size detail at the same distance; only the shorter lens will resolve them as smaller details on the image plane. If you move the subject/target closer, then the angular separation of the details increases. – Steven Kersting Apr 23 at 12:13

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