[hangu]

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Since the medium format lens is bigger, shouldn't more light reach the sensor on a Pentax DSLR?

In case that question is confusing, here's a similar one:
Would a Pentax medium format lens with a max aperture of f4.0 let in more light than a Pentax 35mm lens at f4.0?

Edit:
Since I'm on the stupid questions roll:

If I were to use a 50mm Pentacon 6 medium format lens on a Pentax K-x with the Pentacon 6 adapter, what full frame focal lens distance would it be equivalent to? 100mm?

Another edit:
Has anyone had experience with a tilt adapter for medium format lenses? They're rather expensive for an adapter ($130) but the effects on portraits could be cool, right? Any examples? Thanks!

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Last edited by hangu; 07-16-2010 at 11:27 PM.

[ChaseTheLight]

f4 is f4 and 50 mm is 50 mm. It doesn't matter if it is a MF lens, an LF lens, a 35 mm lens or an APS-C lens. The effect si the same.

[hangu]

f4 is f4 and 50 mm is 50 mm. It doesn't matter if it is a MF lens, an LF lens, a 35 mm lens or an APS-C lens. The effect si the same.

I've always thought that but other people have said otherwise....

[RawheaD]

It gathers more light, but disperses it over a larger area, cancelling each other out. The "F" designation, you can think of it as representing the amount of light that gets transmitted per unit of area. So, like it's been said, an F4 lens is an F4 lens regardless of what size sensor/film you use it with.

[CarbonR]

When you use a lightmeter, you don't set the format. As said, f/4 is f/4. The difference is that it will give a narrower dof if it is used on MF. It is easier to understand when you use several formats. For exemple, my Takumar/6X7 55/3.5 equals to (24x36 equivalent, aperture equivalent regarding to dof only, f/3.5 remains f/3.5) :
- 26/1.7 when used on 6x7
- 55/3.5 when used on 24x36
- 82/5.2 when used on APS-C
For example, if I use the 55/3.5 @f/3.5 on 6x7, I need a 26/1.7 on 24x36 to have the same fov and the same dof, but the lens on 6x7 will transmit f/3.5.
If I use the 55/3.5 @f/3.5 on APS-C, I need a 82/5.2 on 24x36 to have the same fov and the same dof, but the lens on APS-C will transmit f/3.5.
Here we can see the big advantage of the small sensor claimed by APS-C and 4/3 users (and marketers) : use a 50/1.4 lens on APS-C, it will give you a 75/1.4. In fact, it transmits f/1.4, but dof is only f/2 regarding to the lens needed on 24x36 to have the same rendering. We also see the advantage of the bigger format (6x7) : even a slow f/3.5 lens gives an equivalent dof of f/1.7. That's why the 105/2.4 on 6x7 is approximatively equivalent to a 50/1.2 lens on 24x36, but it transmits only f/2.4, whaever the format is (even on a compact cam sensor, where it could be an equivalent of 600/2.4 lens, but f/15 dof equivalent)

[icywarm]

CarbonR's answer was great... but for us math geeks it is something like:


source:wikipedia

so the size of the film does effect the amount of light, but as CR said it is most noticed in DOF...

[Mike Cash]

Historically, the whole reason they started stating apertures in terms of f numbers to begin with was that it was a consistent way of knowing the effect on exposure regardless of focal length (or format, for that matter). Some cameras used to have apertures simply numbered stuff like "No.1, No. 2, No. 3, etc" and while that could be used in conjunction with a particular camera's manual to determine exposure it wasn't horribly useful when going from one camera/lens to another. Going to apertures expressed as a ratio of aperture diameter to focal length (which is what f numbers are) solved the problem. F4 is f4 is f4 is f4.

[Clinton]

- 26/1.7 when used on 6x7
- 55/3.5 when used on 24x36
- 82/5.2 when used on APS-C

When we're talking about different mounts, I think this makes sense, however, there is no difference in the DoF between your full frame film and APS-C. My 85/1.4 has the same amount of DoF in each, just the APS-C is cropped.

[openyourap]

If I were to use a 50mm Pentacon 6 medium format lens on a Pentax K-x with the Pentacon 6 adapter, what full frame focal lens distance would it be equivalent to? 100mm?
examples? Thanks!

Since you're using an adapter, you might consider whether that changes the focal length of the lens. If it does, (say, by moving the lens out from the camera) it's making the image circle bigger (and correspondingly dimmer). Your aperture size hasn't changed, but it's area relative to the focal length has.

You could use Icywarm's Wiki formula to figure it out, or just hit the green button for some stop-down metering.

[Lowell Goudge]

One thing to point out with respect to the concept of gathering more light

As several have said F4 is F4 and an MF lens is no bigger in diameter than an asp-c lens at the front. What is different is the rear elements to allow a bigger image circle, but that image circle does not alter at all the intensity of light hitting any one square mm of film or sensor. All that changes is the field of view when you change from format to format. The "extra" light is really parts of the image that are outside the sensor area and absorbed in all the light baffels in the lens so they do not produce flare

[CarbonR]

When we're talking about different mounts, I think this makes sense, however, there is no difference in the DoF between your full frame film and APS-C. My 85/1.4 has the same amount of DoF in each, just the APS-C is cropped.

Indeed I did not mention it, but these figures are for exactly the same reproduction ratio, so with a 50mm on APS-C, you have to be farther than the same 50mm lens on 24x36, which explains why there is a dof loss

[hangu]

Thanks to everyone for the explanations, I have a better grasp of this now.

Since I'm clearly not the brightest bulb, I want to make absolutely sure I understand this concept. Please tell me if I'm way off the mark here.

I understand the focal length and depth of field changes drastically between medium format and in my case Pentax K-x's APS-C.

A 26mm lens for 6x7 cameras will act like an 82mm lens on my Pentax K-x and 55mm on a Canon 5D full frame. However, I have to use f5.2 on the 6x7 camera to get the same depth of field result as f1.7 on my Pentax K-x, correct? However, the f-stop will still only be f5.2 on the 6x7 camera. To get the same DOF effect as f2.8 on a 6x7 camera, I have to use something like a ~f0.95 lens on my Pentax K-x, right?

If I want the same field of view with a Pentacon 6 lens as my Takumar 85mm on my Pentax K-x, I would need a Pentacon ~43mm medium format lens (probably doesn't even exist), correct? Basically, what kind of a Pentacon 6 lens should I use as a normal portrait lens on Pentax K-x?

If I were to use a Pentacon 6 to Pentax K adapter such as this (click) with an 180mm f2.8 Pentacon 6 lens, would it still be f2.8 on my Pentax K-x? Would I lose a stop or two of light? It'd basically be a 510mm f2.8 lens on my Pentax K-x, correct?

[CarbonR]

You mixed a bit (or misunderstood) what I said. I'll try to be clearer.
"Enlarge factors" :
- 6x7 format : 2
- 6x6 format : 1,8
Crop factor of APS-C format : 1,5

It means that you need a lens 2 times longer on 6x7 (or 1,8 times on 6x6) than on 2'x36 to get the same angle of view (and the lens only needs to be 2/1,8 times slower to get the same dof, for the same reproduction ratio obviously). As the APS-C is smaller than 24x36, you need a lens 1,5 times shorter than on 24x36 to get the same angle of view (and 1,5 times faster to get the same dof). These figures can be combinated : crop factor between APS-C and 6x6 is 1,8x1,5=2,7, which means your 85mm lens on APS-C will act as a 85x2,7=230mm lens would to on 6x6 format. And to equal (in terms on angle of view) a 80mm lens on 6x6 with an APS-C body, you need a 80/2,7=29mm lens (it is normal : 80mm is the stand focal lengh on 6x6 while 29mm is around normal focal lengh on APS-C too).

Edit : to answer about the 180/2.8. As we said, it will still transmit f/2.8. It will remain a 180mm lens (so with a DA 50-200 @180mm, you would have the same angle of view), but you will have the same angle of view of a 180x1,5=270mm lens would have on 24x36, or the same angle of view a 180x2,7=486mm lens (almost 500mm) would have on 6x6 format.
Don't bother with focal lengh and formats. If you know that your 85mm Takumar is enough for you in terms of angle of you, any ~85ish lens, whatever format it is designed for, would do it.

Bonus : A Pentacon 500/5.6 on 6x6 would have approximatively the same angle of view than a 180mm lens would have on APS-C. But in matter of dof, the lens on 6x6 format would be an equivalent of f/5.6/2,7= f/2. So if you use a 500/5.6 @f/5.6 on 6x6, you have a narrower dof than with a 180/2.8 @f/2.8 lens on APS-C, despite you have the same angle of view and despite one is 2 times slower than the other

[alohadave]

It gathers more light, but disperses it over a larger area, cancelling each other out. The "F" designation, you can think of it as representing the amount of light that gets transmitted per unit of area. So, like it's been said, an F4 lens is an F4 lens regardless of what size sensor/film you use it with.

F actually refers to the focal length. The proper notation is f/4. Focal length divided by 4 will tell you the diameter of the aperture opening.

This notation is used because it is consistent across all focal lengths and lens formats. It always works. f/8 on a 100mm lens and f/8 on a 25mm lens will be the same relative size even though the apertures are not the same physical sizes.

[Pentaxor]

When we're talking about different mounts, I think this makes sense, however, there is no difference in the DoF between your full frame film and APS-C. My 85/1.4 has the same amount of DoF in each, just the APS-C is cropped.

are you saying that when you use a full-frame lens on a full-frame camera set at f1.4 at a certain distance and the same type of full-frame lens on an APS-C sensor set also at f1.4 at the same distance will have the same DOF?