crop factor/ aperture/noise......

<p>Just been watching a Tony Northrup video about lenses and he makes interesting watching; I bought a Sigma 60/2.8 for my Nex, thinking to myself that I was getting a nice equivalent to a 90mm lens on a FF camera, which I think is good. I just multiplied the lens by the crop factor and liked the answer and so bought the lens! And I like the lens .<br>

What TN points out (I think) is that the aperture must be altered as well when you have a different size sensor and this is something it seems we have to work out for ourselves, as the camera makers don't help us to see the changes you will need to make. <br>

I think that TN is saying that the camera makers relate the fully open aperture on their lenses to a 35mm frame, whereas for most of us the sensors are smaller and therefore we need to take that into account if we want to consider noise and background blur. The APS sensor in the Nex requires a reduction in ISO by a factor of 2.3X to get the same effect as if it were a 35mm picture.<br>

By which I mean that if two pics were taken, one in a FF camera and one in a Nex for instance, at the same shutter speed, same ISO and same aperture the Nex pic would be underexposed by one stop. I think. Although that doesn't seem correct!<br>

Leave it there I think, I've just confused myself.</p>

<p>Not true. Less light falls on the sensor, but the area of light required is less too, so it makes no difference. The intensity is the same. f2.8 is always f2.8 in light gathering power. There will be a difference in perceived depth of field on the resulting image (due to the different magnification), but the true depth of field will be the same (obviously).</p>

<p>The cropping factor has no effect on the relative aperture (f/stop). You do not have to make an exposure compensation. Nor has it any effect on the depth of field at the same distance from the subject. DOF is related to f/stop and absolute magnification on the film plane. The DOF increases as you move further away to get the same subject coverage in a smaller image area, or use a shorter focal length.</p>

<p>I saw the video as well and while it is interesting information I doubt it matters to most people. I think he was basically explaining how makers of smaller format cameras don't exactly stick to all the scientific rules of calculating f-stops when labeling their lenses as f1.2/2.8 etc. This article on DP review more or less explains what Northrup goes over in his video and helps shed light on how f2.8 on micro 4/3 isn't exactly the same as f2.8 on full frame in terms of total light gathered and also explains why smaller formats will always suffer in low light shooting compared to FF (assuming equal levels of sensor technology). His gripe was how some of these lenses are labeled as 35mm "equivalents" but they are only applying the crop factor to the focal length and not the aperture. The exposure will look the same but it explains why smaller formats will appear noisier at the same settings and how the confusing definition of ISO comes in to play to achieve the same results. I don't think most people care but for purposes of the article and video you need to multiply the f-number by the crop factor as well.<br /> DP review article:<br /> http://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care</p>

<p>I believe this is a reason why the concept of the T-stop was introduced to account for the light transmitting properties of the lens itself [glass] ... the result was that an f/2 lens might be rated a T/2.2 for accurate exposure etc.</p>

<p>I find the way that T-stops have been resurrected for still photography an interesting observation on the way the internet encourages largely irrelevant techno-nerdishness. T stops have always been important for cinematography, it's not like this is new knowledge, but most of us forgot worrying about T stops for stills (if we ever did) when multicoating was introduced.</p>

<p>robin is right, you have greater DoF with an APS-C sensor vs. a FF sensor, all else being equal. it's not underexposure though. and the light-gathering abilities are still consistent regardless of effective aperture. this is one reason that some lens makers are designing lenses which mitigate this to some degree, like the fuji 90/2, or the sigma 18-35/1.8. that sigma 60/2.8 is going to act like a 90/4 in terms of DoF and effective focal length, but still collect light like a full-frame 2.8 lens when shot wide open. so what that means in real terms is you'll get worse background separation for subject isolation purposes but more will be in focus, and your 2.8 will be just as good as a full-frame 2.8 in low light (although the camera sensor will also have an impact here).</p>

Wouldn't the increased enlargement from a smaller sensor

counteract the narrower circle of confusion from the

crop factor?

Suppose you have a full frame camera with a 90mm f/2.8 lens and an APSC camera with a 60mm f/2.8. Suppose they're

from the same generation and the full frame has the usual 1-stop advantage at high ISO. You're in dim light and you're

shooting at high ISO.

 

Which camera do you want to be using? The full frame. Its results will look better because of its 1-stop advantage at high

ISO.

 

Now suppose the 90mm lens were an f/4 but the 60mm is still an f/2.8. Which camera will give better results? Neither -

the faster f/stop on the APSC camera's lens has canceled out the full frame high ISO advantage.

 

So of course high ISO shooting is another field where looking at "equivalent aperture" is useful. If your frame of reference

is 36mm x 24mm, and you're evaluating a lens for a crop frame camera, multiply both its focal length and its aperture by

the crop factor. A 30mm f/1.4 lens on APSC is like a 45mm f/2.1 on full frame. That 42.5mm f/1.2 for M4/3? It's only

equivalent to an 85mm f/2.4 lens on full frame.

<p>It's impossible to generalize between sensors based on size, or even cell size. I am pressed to exceed ISO 3200 with my D3, yet the D4 with 50% more pixels will go higher with good results, and a Sony A7 (24 MP) will broach 12,500 with little or no noise. Face it, technology is improving noise control as quickly as the number of pixels that can be squeezed into a given space.</p>

<p>Secondly, noise seems to increase at a moderate pace with respect to ISO until you exceed a certain threshold. Above that threshold, you have to be desperate for the shot, or the subject is more important than the technical quality. Below that threshold, and it doesn't seem to matter whether the sensor is full-frame or APS-C.</p>

<p>Lenses seem to be improving too, and until recently the resolution of most exceeded that of the sensor by a significant margin. With 24 MP and 36 MP becoming the new "normal", that is changing. In any case, sharpness is generally better in the center of the field than at the edges, and a DX sensor trims away those edges.</p>

<p>A DX sensor presents a decided advantage for use with long lenses, because the focal length is "multiplied" by a factor of 1.5X or so. Most FX camera allow you to crop in camera to DX size, but a DX sensor of comparable quality may have more pixels, hence more resolution.</p>