Tuesday, 4 February 2014

trying to understand normal

One of the things I like (and have blogged about) is a normal lens. In this day of everyone starting photography with a 'standard zoom' (something like 28-80mm or so effective focal length) I reckon that most photographers have lost sight of what is a normal lens.

To me as well as the "normal" definition such as producing a normal perspective, I think its also think of them as being able to take photographs at 'normal' working distances. You know, like when you're in a group of people all standing around normally ...

Back in the day the normal lens that came with a 35mm film camera was typically a f1.8 50mm lens. However by the time digital became mainstream zooms were so entrenched as the norm that people really often never even used a "normal" lens.

So its in this world that I hear people saying things like: "I don't even know what a normal lens is" and "I simply can't see in normal" as well as being confused about the background / working distance / depth of field.

So I thought I'd take the opportunity to present a few images to attempt to explain normal. As well Olympus has recently released a 25mm f1.8 "normal" for micro4/3 (which is probably going to cost something like double the price of equivalent normals for Full Frame) which I don't think will really do what people want. It is an inherent issue with 4/3 (or even APS for that matter).

Since I've been scratching my head about the Sigma 30mm f2.8 "normal" (for micro4/3) which I recently bought I thought I'd compare it to the "standard" 50mm f1.8 that came with my 35mm full frame; as this serves to highlight the "shallow DoF" and also demonstrates the advantage of Full Frame at the same time (and also the limitation of smaller sensor sizes like 4/3).

kirstiHauttalle2The main advantage of a large aperture (aside from allowing faster shutter) is to blur the background either side of the subject. People nowadays tend to think this is done only with telephoto lenses, but it can be done (and until recently was commonly done) with a standard 'normal' lens.

To do this you "open up the lens", but the problem is that its not just the f-number you are after, its the diameter of the aperture. I have an old page on another server here to illustrate this (and the javascript for rollover is something I have not worked out here in Blogger yet).

There in lies the problem, for you see that with smaller formats the focal length of a normal changes. You know a "normal" on Full Fame is 50mm, and a "normal on 4/3 is 25mm, that old effective focal length thing.

So to get that same look as a f1.8 gives on Full Frame on the smaller 4/3 sensor you will need not f1.4, not f1.2 but about f0.9

Anyway it boils down to you need about 2 stops more (larger diameter) on 4/3 to get the same shallow depth of field. Its well discussed on the net (and I've discussed it to here and here).

To give you some numbers:
50mm @ f1.8 = 27.7mm
30mm @ f2.8 = 10.7mm
25mm @ f1.8 = 13.8mm
So the 25mm f1.8 is roughly about half the diameter, which suggests its not going to cut the mustard (and it won't).

So here I am hanging with my mate at the mall. I'm just a step away from him and I take this shot.


I focus on his ears and use f1.8 to render the background soft and out of focus so its clear its a shot of him, not the bicycles or the lady walking towards us ... just my mate on the bench.

Now with the Sigma the background is substantially clearer (the bicycles are now more clear as is the other bench over on the right)


But looking a bit more carefully the focus really is on his face. My bag is clearer as well as the background.
Lets have a clearer look


Yep, same angle of view, but just so much tighter in. Focus is on the face and on the ears.

The message seems to be getting out there however with people looking for a nice shallow normal for their micro43rds cameras.

Panasonic already has a 20mm f1.7 lens which is still a little lacking in the 'shallow normal' area as its equivalent to about f2.8 on the Full Frame.

So Olympus has put their foot onto the dance floor with their new normal a 25mm f1.8 lens. Personally I'm a little disappointed that they didn't make it a f1.2.

There seems to be a lot of interest out there in www land about this, but in my view, I just don't think its going to give the same DoF that a humble f1.8 50mm will give on Full Frame.

I mean don't get me wrong, I expect that it'll be sharp and contrasty lens, but will it be significantly different to the existing lenses?

Will it give you the same rendering as a humble (and half the price) normal on a full frame? Personally I don't think it's going to be the equal.

None of the images I've seen in tests show the new Oly as any significantly better the sigma is at f2.8

But I'm sure they'll sell heaps of them to people who have no idea what normal looks like and check out the images on the Web that just wang on about how good it is. Those sites will show lots of great shots. The approach is to imply you'll take images like that with the new lens.

If you feel like buying the new Oly I'm sure you'll be happy with it especially for size and clarity. But if you're buying it to get a shallow normal, well the bad news is that f1.8 just isn't a big enough diameter and its not likely to jump out at you any better than the Sigma 30mm f2.8 (at half the price).

I thought I'd leave you with a few more overviews ... in the format above, but to save wasting your bandwidth I'll just post the 50mm f1.8 and then the detail comparisons. I'm sorry that the colour match is not perfect between the images as I naturally had to use different cameras to compare full frame and 4/3.

50mm @ f1.8


detail segment


and lastly one that's just a side by side



I guess you can see clearly how quickly the distance becomes obscure even at these sizes. If you like the looks that you can get with a normal lens wide open, then my friend you're really going to have to consider a full frame camera. Still ... its your money.

Oh, and also please right click on any of the above images if you wish to see them bigger

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