Shutter speed cropping and image quality.
May 19, 2019 15:55:34 #
I've been doing some calculations based on a full frame sensor with 36 megapixels
ideally we look for 300PPI for printing 200PPI as a minimum so i figured out some crop factors
1.5x (aps-c)
2x (m43)
3x (around 200PPI)
3x is the max usable if the light is good and the ISO low in practice it may not be usable.
using crop guides in lightroom a 1.5x crop is equal to 4 of the thirds rectangles
a 2x crop is half the width and height (over 300PPI)
a 3x crop is just one of the thirds rectangles. (just over 200PPI)
Lets say we use a 200mm lens then the field of view is 300mm 400mm 600mm
For maximum quality using base base ISO and a moving subject 2/focal length we can calculate the light needed or the aperture needed.
http://mecastronics.free.fr/ExposureLightValueVCam.html is an ugly but useful calculator.
One interesting item is a real focal length or a crop focal length need the same shutter speed. ie with a moving subject at 300mm = 1/600 or a 200mm at a 1.5x crop = 1/600
lets start with a bright sunny day at 14EV at iso 100 and a shutter speed of 1/400th the aperture should be F6.3
with a 1.5x crop raising the shutter speed to 1/600 the aperture is now about f5
2x crop and shutter speed 1/800 f4.5
3x crop and shutter speed 1/1200 f3.6
In the shade i exposed an image at ISO 400 f7.1 1/250th of a second which gave me an ev of 11.7
Ideally at base iso an aperture of f3.6 would be appropriate for that level of light. My lens i was using is f5.6 at 210mm which would give an iso of 250 to go for iso 100 would require a shutter speed of 1/100 with image stabilization thats not too bad in practice (the camera offers 5 stops) in theory i could have used 1/60th as a shutter speed at f7.1 and the camera body would have handled it. Image stabilization doesn't help with subject movement thou.
If I crop to 2x then my shutter speed should be at least 1/400th and an aperture of f2.8 to keep base ISO.
If you are following what i'm saying image quality falls quite quickly in fairly good lighting.
There is one possible improvement by resizing and down sampling you can reduce noise , and a reduction of 50% will still keep the image above 300PPI
This photo was resized using bicubic (sharper) at 50% of course by resizing you lose detail but if thats detail that you can't see , does it matter?
can i make a conclusion from this? Well it seems a 2x crop is almost an acceptable replacement for a longer lens but pretty often we are short of light in a lot of circumstances. Faster lenses will gather more light but at a cost of depth of field.
ideally we look for 300PPI for printing 200PPI as a minimum so i figured out some crop factors
1.5x (aps-c)
2x (m43)
3x (around 200PPI)
3x is the max usable if the light is good and the ISO low in practice it may not be usable.
using crop guides in lightroom a 1.5x crop is equal to 4 of the thirds rectangles
a 2x crop is half the width and height (over 300PPI)
a 3x crop is just one of the thirds rectangles. (just over 200PPI)
Lets say we use a 200mm lens then the field of view is 300mm 400mm 600mm
For maximum quality using base base ISO and a moving subject 2/focal length we can calculate the light needed or the aperture needed.
http://mecastronics.free.fr/ExposureLightValueVCam.html is an ugly but useful calculator.
One interesting item is a real focal length or a crop focal length need the same shutter speed. ie with a moving subject at 300mm = 1/600 or a 200mm at a 1.5x crop = 1/600
lets start with a bright sunny day at 14EV at iso 100 and a shutter speed of 1/400th the aperture should be F6.3
with a 1.5x crop raising the shutter speed to 1/600 the aperture is now about f5
2x crop and shutter speed 1/800 f4.5
3x crop and shutter speed 1/1200 f3.6
In the shade i exposed an image at ISO 400 f7.1 1/250th of a second which gave me an ev of 11.7
Ideally at base iso an aperture of f3.6 would be appropriate for that level of light. My lens i was using is f5.6 at 210mm which would give an iso of 250 to go for iso 100 would require a shutter speed of 1/100 with image stabilization thats not too bad in practice (the camera offers 5 stops) in theory i could have used 1/60th as a shutter speed at f7.1 and the camera body would have handled it. Image stabilization doesn't help with subject movement thou.
If I crop to 2x then my shutter speed should be at least 1/400th and an aperture of f2.8 to keep base ISO.
If you are following what i'm saying image quality falls quite quickly in fairly good lighting.
There is one possible improvement by resizing and down sampling you can reduce noise , and a reduction of 50% will still keep the image above 300PPI
This photo was resized using bicubic (sharper) at 50% of course by resizing you lose detail but if thats detail that you can't see , does it matter?
can i make a conclusion from this? Well it seems a 2x crop is almost an acceptable replacement for a longer lens but pretty often we are short of light in a lot of circumstances. Faster lenses will gather more light but at a cost of depth of field.
May 19, 2019 16:16:06 #
blackest wrote:
I've been doing some calculations based on a full ... (show quote)
Wow. Couldn’t really follow this at all. 2nd time through I just jumped to the last sentence. Summed it up nicely. I’ve always been a cut to the chase kind of guy.
...Cam
May 19, 2019 17:30:50 #
CamB wrote:
Wow. Couldn’t really follow this at all. 2nd time through I just jumped to the last sentence. Summed it up nicely. I’ve always been a cut to the chase kind of guy.
...Cam
...Cam
Well i figure the max crop to use would be around 2x a quarter of the frame, more than that and the resolution is too low.
You pretty much have to keep a shutter speed appropriate to the focal length you are emulating.
Camera sensor sensitivity isn't great and many lenses are too slow to keep at base iso.
One interesting titbit if you crop at the original ratio be it 1.5 2.0 3.0 if you change the aspect ratio it seems to keep the crop ratio.
e.g if i use the rule of thirds guides to crop the side in by a third. if i then select the aspect ratio 2:3 it keeps the width and crops top and bottom to give the aspect ratio and then you can drag the photo to crop to the area you want. You can also change crop guides to suit the photo content. e.g triangles
x3 crop seems desperate and is rarely going to give a result you can be happy with. This shot is pitiful, no sharpness and perhaps the worst thing is it was shot at 70mm and the zoom went to 210mm! What was I thinking ...
May 21, 2019 15:14:19 #
Well, you obviously know what you are do, this is a beautiful capture. Thanks for sharing.
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