When is you f stop not your f stop?

[MadMailMan]

When is your f stop not your f stop?

This may have been discussed before but if the f stop is a function of focal length divided by the aperture diameter. Then surely the f stop is dependent on the sensor size. With a full frame sensor the 50mm you see on the lens is 50mm according to the sensor so the f stop of 2,8 is actually 2,8, right. But on a crop sensor the 50mm on the lens means 50mm x 1,6 (crop factor) 80mm so the f stop ratio can't be the 2,8 as it is on the lens but rather 4.48 (f=80/same diameter as the lens has not changed).

This bring me to a second question. Do the EF-s lenses show the true f stop on the barrels for the 1,6 crop or are we being duped?

Explains why the bokah from a full frame camera is sooooo much better than from a crop.

 

[bwana]

The focal length of the lens doesn't change, your field of view does.

 

[MadMailMan]

But then why do (some) people say that the slowest you should shoot is 1/focal length seconds but when shooting on a crop body the focal length is 1,6 x the lense focal length?

I are confuzzed. :s

 

[MadMailMan]

Aaaaand if it's just the FOV that is different then why do 1,6 crop pictures appear "magnified" instead of just smaller when compared to FF shots. As far as I can tell if you use the same lens on the two (crop and full frame) bodies to get the same magnification, which is governed by focal length you would need to set the lens to say 50mm on the crop but to 80mm on the FF. No?

 

[hayted]

Then why does you not stop!?! Cause I did not see you. (Sorry this is all I can add to this as it is why over my head)

 

[MadMailMan]

Then why does you not stop!?! Cause I did not see you. (Sorry this is all I can add to this as it is why over my head)

It's probably not over your head dude. I think I have just been to bored and overthunked everything and now I'm just as confused as a baby in a topless bar.

 

[hayted]

Lol. There is one I have heard before Have a good weekend and hope you find your answer

 

[bwana]

Aaaaand if it's just the FOV that is different then why do 1,6 crop pictures appear "magnified" instead of just smaller when compared to FF shots. As far as I can tell if you use the same lens on the two (crop and full frame) bodies to get the same magnification, which is governed by focal length you would need to set the lens to say 50mm on the crop but to 80mm on the FF. No?

Because you're using a smaller part of the actual image circle.

 

[koffiejunkie]

But then why do (some) people say that the slowest you should shoot is 1/focal length seconds but when shooting on a crop body the focal length is 1,6 x the lense focal length?

The 1/FL rule is has to do with perceived motion blur, i.e. when you print an image at a reasonably large size, at what shutter speed do you need to shoot for the given focal length not to see any motion blur. Now forget about crop and just think of one camera with two lenses. Lets say a 50mm and 80mm, so bwana's pic demonstrates it nicely. If you look at those two rectangles, the the portion of the image they cover, then think of the inner rectangle and blow up that part of the image so it's the same size as the outer one. Now lets suppose it's video and you're riding on the back of a bakkie. If you watch the video in both those rectangles, which one will shake more?

The inner one, of course, because the distance an object appears to move in the frame is bigger relative to the inner frame, than the outer frame. That is why you need to compensate more when you're shooting crop.

This is, of course, relative. Our habit of looking at things at 100% screws around with it too, because at a very high resolution the motion blur will cover more pixels for the same image as it does at a low resolution.

 

[undesign]

In other words the rule of thumb for handheld shutter speeds of 1/FL refers to 35mm EQUIVALENT focal length, not ACTUAL length? I've always wondered about that.

On a P&S lens of say 50mm with a 350mm equivalent, it intuitively feels correct that the minimum shutter speed should be 1/350, rather than 1/50 - so it makes sense I guess.

 

[MadMailMan]

:wtf:

My brain is fried. It's a quarter to weekend. Aaaand I'm off to Scotland and Northern Ireland for a week, from tomorrow. I'm just going to pretend when I hit the shutter release button that there is an imp inside my picturebox who will paint at the speed of light and I'll get some great shots of the countryside.

P.S. If any of my shots come out all blurry it's not because of Koffie's bakkie it's because I've been drinking too much scotch. Have a good one chaps and thanks for your help with this befuzzelment of mine.

 

[bwana]

In other words the rule of thumb for handheld shutter speeds of 1/FL refers to 35mm EQUIVALENT focal length, not ACTUAL length? I've always wondered about that.

On a P&S lens of say 50mm with a 350mm equivalent, it intuitively feels correct that the minimum shutter speed should be 1/350, rather than 1/50 - so it makes sense I guess.

Yes but it's a very rough guide as far as I'm concerned.

 

[koffiejunkie]

P.S. If any of my shots come out all blurry it's not because of Koffie's bakkie

I don't have have a bakkie. Too heavy on petrol...