[BBear]

Not sure if the info I read is correct.

For instance, for a crop factor of 6 (compact camera):


6.3mm f/3.5 is equivalent to 38mm f/21


I understand the focal length in regards to crop factor, but I didnt know the same rule applies to aperture.


Can someone clarify this?


Thanks!

[Talisker]

6.3mm f/3.5 is equivalent to 38mm f/21

No. A 6.3mm f3.5 is always a 6.3mm f3.5. Its field of view might be equivalent to that of a 38mm (or whatever) on a 35mm camera but that has nothing to do with the aperture.

[Gooshin]

No. A 6.3mm f3.5 is always a 6.3mm f3.5. Its field of view might be equivalent to that of a 38mm (or whatever) on a 35mm camera but that has nothing to do with the aperture.

not quite correct,

aperture is a product of focal length

F-stops, focal length and lens aperture


given the formula,

3.5 = 6.3 / X

X = 1.8mm

this 6.3mm focal length lens has its largest opening at a mere 1.8mm!!!


a 1.8mm opening on a 50mm lens would equate to an F stop of 27!!!

X = 50/1.8
X = 27


so the OP is technicaly correct, but like always people get the terms wrong


OP: you basicaly have a camera that has the field of view of whatever a 38mm lens would have on a full frame camera (what that is in degrees i dont know)

and you have your largest diaphragm opening at 1.8mm.

use that information as you will.

[falconeye]

For instance, for a crop factor of 6 (compact camera):
6.3mm f/3.5 is equivalent to 38mm f/21

Gooshin is correct. But be careful, though...

- For field of view, apply your crop factor.

- For depth of field, apply your crop factor
(because, as I had pointed out in another thread, depth of field is basically a function of only two paramaters: field of view (in degrees) and absolute aperture (in mm)). -- and in order to compare absolute apertures, you apply your crop factor as Gooshin had pointed out.

- For exposure settings, don't apply your crop factor, though!

[BBear]

OP: you basicaly have a camera that has the field of view of whatever a 38mm lens would have on a full frame camera (what that is in degrees i dont know)

and you have your largest diaphragm opening at 1.8mm.

use that information as you will.

Honestly I am not sure about the calcs you performed, but I think somehow you confirmed the reason why compact cameras (big crop factors) have a HUGE depth of field, and ALWAYS have everything focused properly.

Can you just confirm in short ways what is the equivalent of a FL 6.3mm f/3.5 in 35mm format? (assuming a crop factor of 6)



Thanks!

[BBear]

Gooshin is correct. But be careful, though...

- For field of view, apply your crop factor.
- For depth of field, apply your crop factor
- For exposure settings, don't apply your crop factor, though!

What are you trying to say here? The field of view I understand. I am having problems understanding what you guys are saying in regards to DOF x Aperture x Crop Factor.





Maybe I should change the question:

Let's say I want to reproduce a picture taken with a 6x crop camera 6.3mm @ f/3.5 in a regular film SLR.

Which lens and what settings should I use?

[Gooshin]

Maybe I should change the question:

Let's say I want to reproduce a picture taken with a 6x crop camera 6.3mm @ f/3.5 in a regular film SLR.

Which lens and what settings should I use?

you would need a 38mm lens shooting at F21 on a 35mm SLR

and my calculations are correct, i used a calculator, if you think my equations are wrong, you can write to the author of that website and tell him that hes a hoot.

[BBear]

you would need a 38mm lens shooting at F21 on a 35mm SLR

and my calculations are correct, i used a calculator, if you think my equations are wrong, you can write to the author of that website and tell him that hes a hoot.

Thanks!

I wanted to find out the "equivalent" in 35mm film.


Regarding your calculations, I didnt say it was wrong! I said I didnt fully understand how you got to it.. I am trying to reverse engineer it to understand what is going on.


And yes, the original formulas/numbers in the OP was my guessing from what I read a while ago. It could be off! ( and it is! )


Thanks again for the clarification... maybe if you had time to explain how to find out the "equivalent" aperture would be nice!


BB

[BBear]

6.3mm f/3.5 is equivalent to 38mm f/21

you would need a 38mm lens shooting at F21 on a 35mm SLR

Wow.. i just realized that your numbers matched what I guessed originally!

so yeah... all you do is multiply both aperture and FL by the crop factor! I thought you had performed some crazy LOG calculations or things like that!


Cool!

[falconeye]

if you think my equations are wrong, you can write to the author of that website and tell him that hes a hoot.

Hi Gooshin. No reason to quote a website here. Apertures are named f/5.6 (for instance) BECAUSE this already IS the equation! (E.g., with e.g., f=50mm, the aperture is 50mm/5.6 or 8.9mm)

What are you trying to say here? The field of view I understand. I am having problems understanding what you guys are saying in regards to DOF x Aperture x Crop Factor.

You basically have sorted it out already. As far as Field of View and Depth of Field are concerned.

However, there is a third consideration.

If you multiply both, the focal length AND the f-stop by the crop factor, you must ALSO multiply the ISO setting by the square of the crop factor (otherwise, the image will be terribly underexposed).

So, an ISO50 image, with focal lengths multiplied by 6, becomes an ISO1800 shot.

Now, you have a perfect imitation of the P&S image: Exactly the same Field of View, Depth of Field and Noise. The noise is the same because the sensor captures more light by a factor of the square of the crop factor which makes the pixel noise equal.

If however, you don't apply the crop factor to the f-stop, you end up with less noise and less Depth of Field.

The DSLR lets YOU make the choice (between shallow DoF and noise) whereas with the P&S, there is no choice.

[BBear]

If you multiply both, the focal length AND the f-stop by the crop factor, you must ALSO multiply the ISO setting by the square of the crop factor (otherwise, the image will be terribly underexposed).

So, an ISO50 image, with focal lengths multiplied by 6, becomes an ISO1800 shot.

Now, you have a perfect imitation of the P&S image: Exactly the same Field of View, Depth of Field and Noise. The noise is the same because the sensor captures more light by a factor of the square of the crop factor which makes the pixel noise equal.

If however, you don't apply the crop factor to the f-stop, you end up with less noise and less Depth of Field.

The DSLR lets YOU make the choice (between shallow DoF and noise) whereas with the P&S, there is no choice.

perfect detail and explanation about the ISO. i tink this close all questions.

thanks!!

[Lowell Goudge]

Wow.. i just realized that your numbers matched what I guessed originally!

so yeah... all you do is multiply both aperture and FL by the crop factor! I thought you had performed some crazy LOG calculations or things like that!


Cool!

just remember there is the apature SIZE and the F-Stop

F-Stop, (what mose people refer to as apature) is the ratio of focal length to lens opening.

If you have an F3.5 lens it means that the focal length is 3.5 times the apature diameter. That is all.

Now, when you apply the crop factor for conversion from one format to another, and say multiply the focal length by 6, to use the example above, to get the same F stop, you would need an apature opening 6 times the diameter.

Now, if you consider that a lens 6 times the focal length multiplies the image size by 6 (assume you are a long way from subject) this also means it spreads the image 6 times the length and width (36 times the area) you need therefore 36 times the opening (area) in the lens also to keep exposure the same, but when you multiply diameter by 6 you get 36 times the opening area, hence exposure is constant providing the new lens is still F3.5

Make sense?

[BBear]

Now, if you consider that a lens 6 times the focal length multiplies the image size by 6 (assume you are a long way from subject) this also means it spreads the image 6 times the length and width (36 times the area) you need therefore 36 times the opening (area) in the lens also to keep exposure the same, but when you multiply diameter by 6 you get 36 times the opening area, hence exposure is constant providing the new lens is still F3.5

Make sense?

No it does not mas any sense!


You are confusing me now.. i thought i had it all figure out:


6.3mm f/3.5 @ ISO 100 = 38mm f/21 @ ISO 3600

[Jodokast96]

I think it's more like this:

6.3mm f/3.5 = aperture size of 1.8mm


So for a 6x crop factor you get:

(6.3mm x 6) = 37.8mm f/3.5 (this doesn't change) = aperture size of 10.8mm

or

1.8 aperture size x crop factor of 6 = 10.8mm aperture size.

[straightshooter]

I knew I shouldn't have read this. Math is the reason I was a liberal arts major.

OK, I understand about focal length, aperture, etc. but what the heck is the crop factor. Is that the .5 you apply to an older lens to get the apparent focal length for a DSLR compared to a SLR?

Now I'm going to get 2 aspirin for the headache this thread gave me.

CW