Save & earn with MPB; trade-in and buy pre-loved



Time for an update: I still use film, though. Not vast quantities, but I have a darkroom, and I'm not afraid to use it.

I enjoy every image I take: I hope you'll enjoy looking at them.
...Read More


29 Jun 2021 9:59AM   Views : 418 Unique : 260

I always include the Inverse Square Law in the lighting workshops that I run – I think physics is important, although some politicians know that Latin and Greek matter more (they’re wrong: gravity goes downwards even if you tell it not to in an ancient language. But to lose sight of the past is to take risks with history). And today, courtesy of Altimages, a little bit of algebra.

I blogged about a downside of film – he posted a link to an article about ISO-invariant sensors. Now, this is interesting, in terms of one or two specific situations when you might want to make use of what’s in the article, and also for those who really want to understand how their cameras work (I mean REALLY understand, in depth and detail). For most of us, it raises only one or two things to be careful about.

The gist of the article is that some sensors give very much the same results regardless of the ISO setting and exposure, and the author explains why and how. The statement is true within parameters, and there are considerations that affect how you might apply it in your pictures. And it’s also made clear that it may have no practical effect on what you are doing… Indeed, right at the start, the article suggests that many readers may want to skip to the last section, which gives practical advice for – frankly – almost all of us.

The article hinges on the fact that all digital cameras do things that amplify the electrical signal that light striking the sensor produces, and they do it at different stages between photons hitting the sensor and recording digital code on your memory card. Problems can arise at each stage, and some are inherent in the system: manufacturers work hard to design them out, and their success leads to ISO-invariance.

And the algebra? I’m sure there’s a lot involved in the design work, but the crucial issue here is that it matters when (at which stage in the proceedings) you do the multiplication. In most equations, it’s fine – the order of doing things doesn’t matter, nor does the order of writing them down. X times Y is the same as Y times X, and it’s purely a matter of convenience which way round you do things – spotting that’s one of the tricks of being good at manipulating equations.

But, in some circumstances, the order of operations is crucial (memory says). Sometimes, putting the cart before the horse is fatal to a solution, rather than merely inconvenient (have you ever seen anyone trying to get a horse to push a cart? Photographs, please!) And it’s the same with ISO. The camera amplifies the signal digitally, later in the process, and there’s an analogue amplification process earlier on. If changing the ISO setting depends on the analogue stage, the world is unchanged: if it’s done at the later, digital stage, you might as well do it in editing.

There are practical consequences that mean you probably need to know which ISO settings are ‘native’ and which are extended – it affects dynamic range. And none of it matters if you only shoot JPG images. For the rest, go and read the article. It’s repetitive in places, but that’s good with a complex subject, because you keep forgetting things. Repetition gets it into your neurons…

Recent blogs by dudler

Focus scales

If you’ve been taking pictures since before autofocus arrived, you’ll be very familiar with focus scales – they are one of the primary controls on an old-school camera, and just one more of the things that you really needed to get right. With autof...

Posted: 27 Dec 2022 7:01AM

Porcelain processing

People commented on the look in my last post and it seems like a good idea to share the secrets for Christmas. I learned the technique several years ago: a model’s boyfriend told me about it, and a website that described it in detail: I tried it, l...

Posted: 23 Dec 2022 10:47AM

You develop your own films don’t you?

If you have your own darkroom, or if you use film cameras regularly, there are always a few people who mention the attic. As in ‘Grandpa’s cameras are in the attic. I don’t even know if they have film in them!’ This leads me to ask if I can have a l...

Posted: 16 Aug 2022 11:17AM

Choose your pond

There’s an old saying about being a big fish and a little pond. Do you want to be the most important person in a small organisation, or are you content being a relatively small cog in a big machine? It’s the same in photography. With relatively mo...

Posted: 3 Jun 2022 2:25PM

Graduated filters

This is for Hannah, and anyone else who has come across the casual way that a lot of togs talk about one or two types of filter that landscaper photographers use a lot: graduated filters and neutral density filters. A graduated filter is one that i...

Posted: 25 Apr 2022 12:18PM


dudler Avatar
dudler Plus
20 2.1k 2048 England
29 Jun 2021 9:59AM
No pictures...

I may add some later, when I've had time to experiment with a camera or two...
AltImages Avatar
AltImages 3 4
29 Jun 2021 10:52AM
I'll try to offer less distracting suggestions in future John! 😊

Here's another one for you to ponder. The inverse square law doesn't always seem to work!!! Years ago when I first shot in my derelict room I used a tripod and lovely available light from the window and balanced it by partly opening the door on the other side of the room. But all that changed when I put a piece of translucent plastic decorating dustsheet over the window to stop the window cleaner looking in! And then the quality of light tumbled. Obviously the further from the dust sheet light indeed falls off according to the inverse square law, But I reckon that without the sheet in front of the window there was also an element of parallel light rays coming straight though from afar that made a big difference. Trouble is, as a scientist, I can see both sides of the argument as being the true answer!

Finally, the horse before the cart. It reminds me of the local farmer who delivered our milk in a horse and cart. Certainly his horse could back up to do a three point turn in the road! Lol
dudler Avatar
dudler Plus
20 2.1k 2048 England
29 Jun 2021 11:48AM
A three-point turn is not the same as steady progress. Ever tried reversing along a road at speed, when it's not straight?

I think you're right, in that putting up the screening plastic altered the path of the light rays significantly, and that before that, you had a distant 'spotlight' shining in. The plastic scattered the light all over hte place, and effectively became the source.

the proof is in the exposure readings - or, at least, data that should inform the theorising...
Acancarter Avatar
29 Jun 2021 8:39PM
Thanks John, good blog and link to the article on iso invariance. I’m going to have to look at this in a bit more detail. The noise from a simple photo receiver has a bandwidth aspect - the noise reduces the longer you integrate the signal, by the square root of time. So I’d expect at some level iso invariance would depend on the shutter speed, as the different sources of noise would have a different frequency dependence, (often termed Noise Spectral Density). True photon noise will be ‘white’ ie flat NSD whereas read noise and other noise sources won’t be -they will have increasing noise at low frequencies (1/f noise) and maybe noise that peaks at higher frequency to enable faster read out. Putting this together there might just be something like ‘iso invariance reciprocity failure’. I thought I’d left this stuff behind on retirement but you made me think, which can’t be bad! Andy
dark_lord Avatar
dark_lord Plus
19 3.0k 836 England
29 Jun 2021 9:26PM
My brain switches off where maths is involed.
That said I can understand increased noise when there is a numerical result of 2-0 in northwest London Smile
AltImages Avatar
AltImages 3 4
29 Jun 2021 10:39PM
I'll let you know about the light readings in due course John. I'll also compare a grey card pointed at the window with my trusty (very) old Weston Master II plus invercone. As I suspect that the incident light reading will decrease because of increased wall vs window, whereas the grey card will be a true 'reflection' so to speak.

You must be a member to leave a comment.

ePHOTOzine, the web's friendliest photography community.

Join for free

Upload photos, chat with photographers, win prizes and much more.