Effective "f" number

[lerabu]

I am looking into buying a 50mm lens for my camera and there are is one aspec I was thinking about.

When I read "Understanding Exposure" by Bryan Peterson he mentioned at the start of the book that in some P&S cameras their smallerst aperture would be an equivalent of f/64 on film camera beacuse of the size of the sensor being taken into account.

Now, my D80 has a 1.6 crop factor when compared to and FX camera (multiplying the effective focal length of the lens by that factor).

Now thinking of the construction of the lens, when I mount an FX lens (such as the 50mm 1.8D I am looking at buying) would the widest aperture effectively be 1.8 divided by 1.6 the crop factor because in percentage terms "much more" of the sensor fits into this aperture than a full frame sensor?

Or did I get it completely wrong?

[jdepould]

No. You just go by the actual number.

[sk66]

I think you misunderstood somewhat. Aperture number is a ratio of opening diameter to focal length so a 50mm f1:2 (f/2) lens means the aperture will open up to 25mm. This is always the case for interchangeable lenses.

The confusion with P&S is because they always state the focal length of the lens as the "effective focal length" which is based upon sensor crop factor. I don't know if they "adjust" the stated apertures for that, but it wouldn't make sense to do so.

This is not a consideration for DSLR's...

[Elmo]

I did not go back and look (and my copy is film based) but I suspect the equivalent thing was referring to Depth Of Field (DOF). Original DOF calculations were based on a concept called Circle Of Confusion. This is the maximum diameter of a circle that the eye will recognize as a point and its size is based on print viewing distance.

With digital the COC has become the pixel size. As the pixel size is much smaller than a normal COC the apparent DOF is much much larger. As sensor size gets larger the pixel size becomes larger and the DOF becomes smaller.

Note: pixel density also effects pixel size and DOF but including it in the above add much more to the confusion.

[inkista]

+1 what Elmo said.

Just as crop factor is about field of view, and not focal length, the issue you're discussing is about DoF and not aperture.

The aperture's f-number is always going to be the ratio of the focal length of the lens to the diameter of the aperture opening:

f_number = focal length / aperture_diameter

f/2.8 lets in the same proportional amount of light on a P&S camera as f/2.8 on a crop-body dSLR, as f/2.8 on a full-frame, as f/2.8 on a medium format, and as f/2.8 on large format, etc. etc.

Another reason you have a huge doF with a P&S camera is because the sensor is so small, the focal lengths of the lenses used on them are equally small. While a P&S camera may say it has a 28mm-105mm (35mm film format equivalent) focal length, in reality, it may have only have a 5mm-40mm lens. You essentially have a 5x or 6x crop factor with most P&S cameras.

And the shorter the focal length of a lens, the deeper your DoF will be. On my dSLR, if I use my 8mm circular fisheye lens, even if I'm shooting with it wide open at f/3.5, I can still get everything from a foot and a half in front of me to infinity in focus.

[ravncat]

Yes, equivalent depth of field changes, but the amount of light coming through is the same

So my Olympus ep-1 with its micro four thirds sensor, appears to have more depth of field (About twice as much) than my full frame nikon for the same framing, but the exposure for the F stop is still the same.

The depth of field change is tricky to wrap your head around at first, and that is because there is an assumption in play, along with an entangled principle.

The assumption is that image framing is the same which means different perspectives and distances
The principle is that dof is also related to magnification of the image

As your sensor size decreases, the depth of field will increase for a given aperture (If and only if you are filling the frame with a subject of the same size and distance). If you`re using a larger sensor, you`ll have to get closer to the subject - or use a longer focal length in order to fill the frame. So this implies that one has to use smaller and smaller apertures to maintain the same depth of field on larger sensors, or larger and larger apertures for smaller sensors.

This also means the following, Depth of Field will appear to change if you crop and then magnify the cropped part of the image to be full image size. Freaky huh? There are other implications but they may be a bit too confusing to add at the moment.

[TheEagleSpirit]

I own a Nikon D90 and D7000 with f2.8 and f2 lenses both zoom and fixed 60mm f2. I have studied a book called mastering the D90 that gives a lot more detail in ISO, depth of field, shutter speed than my actual camera guide goes into and still get confused. I used to shoot in aperature mode mostly but started shooting in manual mode on the mode dial of the camera using the exposure meter. I am getting better pictures since going to the manual mode and occasionally bracketing if I am not sure of tricky lighting since I also don't use flash much. Since reading your posts on this particular issue of F-stops and DOF it brings me to understanding a few things more clearly. The ISO is also a big factor when it comes to lighting and shutter speed. I love experimenting and all your ideas are very useful. Thanks to all of you.

[lerabu]

Thank you all for the answers!

The first time I read them all you all sent my brain down confusion highway but the second time around things were clearer

It appears I may have phrased my question wrongly but indirectly I got part of the answer to my question, so I'll try to forward the answer based on the conclusion from your answers above.

I understand that the aperture is related directly to the focal length and that is clear and it is also clear that it is lens specific, thus f5.6 at 100mm will always be the same. - Check!

I understand now the DOF explanation, even though the pixel density part is clear but yet confusing.

What I was actually alluding to the most in my question is the following: When a lens is set to a certain aperture (f2 for sake of argument), there is a given amount of light that comes through at a certain intensity (quantity of light per time unit if you like) which determines the shutter speed. My question is, if my sensor is smaller (crop factor) does the shutter speed reduce (faster lens) because I am using a full frame lens on a cropped sensor, actually, is there more light on my surface area of the sensor due to the way that the lens is channeling light to towards the sensor, or is the surface area and amount of light ratio always equal no matter what the aperture is?

I am asking this because I know that DX lenses are built differently and focus onto a smaller area adapted for the cropped sensor. When you mount a DX lens on an FX camera, only the number of pixels which "reconstitute" the cropped sensor are active. So an FX lens would in absolute terms let in more light, but the ratio of light amount per surface unit stays the same I presume?

This is just basic physics actually, but I am trying to ascertain whether an FX lens will be faster on a cropped sensor than a DX lens of the same specification, I guess the answer is, they will be equal?

lol, confusion rollercoaster!

[RichardTaylor]

The sensor "sees" certain amount of light.
The amount it "sees" is determined by the aperture (how much) and by the shutter speed (for how long ).

The size of the sensor has nothing to do with it.

However how well it can see the light does depend on the sensor/camera body and lens image quality.

[lerabu]

Originally Posted by ravncat

Yes, equivalent depth of field changes, but the amount of light coming through is the same

So my Olympus ep-1 with its micro four thirds sensor, appears to have more depth of field (About twice as much) than my full frame nikon for the same framing, but the exposure for the F stop is still the same.

The depth of field change is tricky to wrap your head around at first, and that is because there is an assumption in play, along with an entangled principle.

The assumption is that image framing is the same which means different perspectives and distances
The principle is that dof is also related to magnification of the image

As your sensor size decreases, the depth of field will increase for a given aperture (If and only if you are filling the frame with a subject of the same size and distance). If you`re using a larger sensor, you`ll have to get closer to the subject - or use a longer focal length in order to fill the frame. So this implies that one has to use smaller and smaller apertures to maintain the same depth of field on larger sensors, or larger and larger apertures for smaller sensors.

This also means the following, Depth of Field will appear to change if you crop and then magnify the cropped part of the image to be full image size. Freaky huh? There are other implications but they may be a bit too confusing to add at the moment.

Thanks to all for the answers again. I re-re-re-read and all answer but this is actually what I was looking for

Lol took me a while to udnerstand all the answers XD

Thanks all!