Is f2.8 lens equal to f4.0 lens at the same aperture?

[cbjetboy]

Are my EF 70-200mm f/2.8 L IS USM lens and my EF-S 17-85mm f/4-5.6

IS USM lens the same (speed-wise) if I have the aperture set at say

f/4.0 for example or is the 2.8 letting in more light? I ask

because I want to increase my DOF with a higher aperture value but I

am shooting in low light such as a church. So if I have the camera

set, for example, f/4.5, 1/70, 85mm, ISO400, AWB....will both lenses

be equal in terms of speed? Am I asking this question correctly?

Hopefully someone will understand. Thanks!

[cbjetboy]

One clarification...technically I'm sure the EF-S lens I specified will mess that question up so use the EF 24-105mm f/4 L IS USM lens instead so the conversion factor won't be a factor in the question. Thanks!

[james_lai]

The amount of light coming through the lens will be the same.

 

Other qualities of the lens will affect how your image looks so you might want to try both lenses to see which you prefer at those settings.

[lovcom_photo]

The F-stop is a ratio, so for example F4 on a 100mm lens is smaller then F4 with a 200mm lens.

 

It's the focal length divided by the diameter of the aperture size.

 

For a 100mm prime, aperture diameter of say 50mm is F2, 25mm is F4, 12.5mm F8 (100/12.5 = 8).

 

That is way the smaller the F-stop number, the bigger (wider) the aperture.

 

Again, aperture F2.8 for the Canon 24mm prime is smaller then the F2.8 aperture of say a 600mm Lens.

 

F-stop X for one lens is not necessarily the same size as same F-stop at another focal length. It's about ratios.

[ken munn]

Chad,

 

Whatever the maximum aperture or the focal length of the lenses you compare, at the

same aperture and the same shutter speed, the same amount of light will reach the film,

or the chip.

 

A f2.8 lens will only allow more light through to the film than an f4 if it is set to a wider

aperture than f4, such as f2.8 or f3.5. Apertures like f2.8 or f4 are not consistent with

good DOF, you'd be better shooting at f8, f11, or f16.

 

If you want to shoot in a church where the light is not strong, but with a large DOF, you

need to use a smaller aperture (which, bizarrely means a larger f number!) and either

increase the film speed (or the digital equivalent), or choose a slower shutter speed (or

both). Slower shutter speeds call for some outside help, either a support like a tripod or

other form of rest, or image stabilization inbuilt with the lens.

[lovcom_photo]

Another way to fatten up depth-of-field without stopping down is to back up...get further away from your subject as required or if you're able to.

[robert_hall2]

Answer: the amount of light coming through the lenses should be the same. Question to anyone: how do lens focal length and aperture relate? A 28mm lens at f4.0 will have more depth of field than a 85mm lens at f4.0. Why? (I used to know, but forgot. Thanks in advance.)

[michael_madio]

For clarification, "lens speed" is simply another term for maximum aperture. At the same aperture, they both let in the same amount of light. An aperture of f/2.8 will let in more light than an aperture of f/4. To increase DOF, you need to reduce your aperture. A smaller aperture means a slower shutter speed, which can be countered by higher ISO or flash.

[lovcom_photo]

To increase DOF, you don't necessarily have to stop down...agin, you could back up if the composition allows.

 

The three things that effect DOF:

 

Aperture size.

 

Focal Length.

 

Distance from Subject.

[m_barbu1]

Robert, <a href="http://www.google.com/search?q=depth+of+field+calculator">These links</a> might help you with that.

[ifeito]

Dan,

 

Hate to say this, but the three things that AFFECT dof are...

 

Just to add one more example to Chad, two zoom lenses set at the same focal lenght (say 70 mm on both) set at the same aperture (say f/4.0 on both, will give you the same diameter in the diaphragm and thus the same amount of light.

 

It's interesting that the name "stops" comes from the days when there wasn't any diaphragm, but a set of plates with progressively smaller holes in them. The plates were made so each plate had half the area of the previous one so you could calculate exposure times of 2X from the previous plate. That's why the f-stop numbers are multiples of the square root of 2. That's 1.4 = 1.4142; 2.0; 2.8 = 1.4 x 2; 4.0, 5.6, 8.0, etc.

 

Ignacio

[lovcom_photo]

Affect, effect, dont splet heres wuth mee.

[steve_dunn2]

<p>To get really nitpicky, chances are that they don't let in exactly the same amount of light. Close, but not identical. This is why cinematographers use t-stops, not f-stops, to set exposure. Rather than measuring the diameter of the aperture and comparing it to the focal length, t-stops measure how much light actually comes out the back of the lens.</p>

 

<p>There is some light lost along the way through the lens; in general, the more elements there are, the more light doesn't come out the other end. As well, if you're comparing two otherwise similar designs but one has more effective anti-reflective coatings, there may be a difference in how much light comes out the other ned.</p>

 

<p>But to a reasonably good approximation, if you set your 70-200 and 17-85 and 24-105 and 85/1.8 and 85/1.2 to the same focal length and aperture, you'll get about the same amount of light out of the back end of each one.</p>

[ifeito]

No problemo!

[skipd]

Chad, something that I didn't see in all the replies:

 

While the two lenses you list would provide the same exposure if both were set to f/5.6, for example. Obviously, the f/2.8 lens could be used in a lower light situation by opening it up more than the other lens could be.

 

The thing that wasn't mentioned above is that the f/2.8 lens will ALWAYS give you a brighter viewfinder image than the slower lens. This is because while you are viewing and focussing, the lens is at its maximum aperture (unless you are pressing the DOF preview button).

[farmer on the hill]

You will find that the focusing of the 2.8 is more accurate than the f 4 lens. All auto focus lenses focus at the widest apperture and then stop down to the selected apperture as the shutter goes, the reason being that with more light the more information there is to work with focusing. If you were to use a light meter and it were to give you values of lets say iso 100 125 sec. f 8. this will hold true for whatever lens you use for that particular shot.

[steve_dunn2]

<p>In addition to the benefit of more accurate AF which comes from having a faster lens, some bodies also have higher-precision AF sensors which are activated by faster lenses. For instance, the central sensor on the 20D goes into high-precision mode if the lens is f/2.8 or faster.</p>