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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.
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3 Nov 2020 9:36AM   Views : 446 Unique : 293


This blog follows on from one of Professor Bob Newman’s columns in Amateur Photographer, about the idea that smaller sensors give greater depth of field, which he started by saying is ‘not exactly’ true. He then went on to discuss macro lenses, and it’s here that the nugget – in my eyes, anyway – was concealed.

Now, I have the impression that Professor Bob Newman’s monthly technical articles in Amateur Photographer are sometimes a little confusing. Certainly I find them so, and I believe this is the result of three things. First, Prof Newman actually knows a lot of stuff. This means that he wanders easily into the long grass of physics, in a way that leaves the rest of us wondering what just happened. But he writes about interesting things, and important ones, so I find it's worth the effort of persisting, even if i come out knowing something new that is not the something new I was expecting to know.

And he’s a controversialist. That means he often asks a contentious question, just to make a point – and, in my judgment, he doesn’t always answer the question so much as pose another one! And finally, he’s a competent rather than great writer of the language.


Anyway, three-quarters of the way through the article is something that is absolutely obvious, but which I hadn’t thought about. Macro lenses are sold on the basis that they give one-to-one reproduction. Irrespective of focal length, they cast an image on the sensor that is precisely the same size as the real object. A one-inch object will occupy an inch of sensor.

Ahhh… There’s the thing. If you’re comparing full frame with micro four thirds, an inch of sensor changes from ‘fitting comfortably’ to ‘won’t fit’.

So, to get as (apparently) close with my Olympus OM-D as with my Alpha 7, I need only go to 1:2 to fill the frame as well as the alpha 7 does at 1:1. Simples…

But the article was about depth of field, and that may take me a while longer to sort out in my mind. Follow this space.


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dudler Avatar
dudler Plus
20 2.1k 2048 England
3 Nov 2020 9:39AM
Top is the view with an Alpha 7 and a Sigma 105mm set to 1:1 reproduction. Bottom is from an Olympus OM-D EM-1 with a 60mm Olympus lens, also set to 1:1.

For speed, both were handheld, at very high ISO: I do NOT recommend this for quality! In the middle is a handheld shot of some stuff sprouting on a log in the garden, also hand-held wit the Olympus, and which set my thoughts on a train of macro...
AltImages Avatar
AltImages 3 4
3 Nov 2020 12:07PM
I disagree with Prof Newman. I'm pretty sure that smaller sensors always produce greater depth of field. That's why mobile phone photos are sharp back to front even when close up yet have a fixed aperture of around f1.7. It reminds me too about a photo I took on a small sensor compact camera (Fuji f31fd). I was mowing the lawn and saw a tiny mushroom. It was no more than an inch tall. So I got my little camera, added a £20 magnetic wide angle converter from ebay to the front and took this photo at f8 according to the exif data. And you just wouldn't have been able to take this photo this way with a 'proper' camera! https://www.dropbox.com/s/7d6gq8y4pf1abew/d10qwtk-a6f4f6cb-04fc-4161-8c52-b8ffaaa53cf2.jpg?dl=0
dudler Avatar
dudler Plus
20 2.1k 2048 England
3 Nov 2020 1:13PM
I'm still working out what the article was actually saying... If anybody wants to see what it says, I'm happy to email a scan - just send me a private message to say where to send it.

My instinct is very much that DoF is, in all practical terms, greater with smaller sensors. But there's that phrase 'not exactly' - and it may hold many cans of intellectual worms.
GGAB Avatar
GGAB 7 31 1 United States
3 Nov 2020 1:36PM
The depth of field can be determined by focal length, distance to subject, the acceptable circle of confusion size, and aperture.
Since focal length is part of the equation for figuring depth of field, it would make sense that sensor size would have an effect.

You tweaked my interest with the discussion.
I found this article that goes to great length to explain the phenomena:

dudler Avatar
dudler Plus
20 2.1k 2048 England
3 Nov 2020 1:50PM
George, I think that there's another factor that you've mentioned that may work the opposite way - the acceptable circle of confusion is smaller when the sensor is smaller, for a given number of megapixels.

I shall read that linked article, and see if that helps increase my understanding.
dark_lord Avatar
dark_lord Plus
19 3.0k 836 England
3 Nov 2020 4:58PM
Interesting ideas.
We now have another month with not much else to do but experiment Smile

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