Sensors and Depth of Field

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DoogsATX
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Re: My camera

Post by DoogsATX »

It's both a simple and not simple at all thing. Will weigh in later when I can get on the computer...
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Re: My camera

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Okay...knowledge-dropping time! To caveat - I'm not a pro photographer. I've been an enthusiast for going on 20 years now, and in the course of my career I've had to get seriously smart about the technology behind it all (once upon a time I wrote educational articles/videos/etc for Sony...mostly about photography).

First, the lay of the land...

Full-frame image sensor = an image sensor the same size as old 35mm film. Approximately 36mm x 24mm.

Crop sensor or APS-C size sensor = an image sensor the same size as APS film, so closer to 23mm x 15mm.

When DSLRs first arrived on the scene, they used APS-sized sensors because, well, they were smaller and therefore cheaper to produce. Over time, a few full-frame sensors were introduced at the bleeding edge. Now, they're trickling down into the prosumer level (like my Nikon D610...which is basically a D7100 body with a bigger sensor shoved inside).

Crop factor = the size differential between image sensors. Basically how much of an image is "cropped". The crop factor between full-frame and APS is 1.6x. Like so.

Image

What this means in real life. On a full-frame camera, a 50mm lens acts just like a 50mm lens. On an APS-sensor camera, the crop factor reduces the field of view, making the 50mm behave functionally like an 80mm lens. To get that "50mm + full-frame" field of view with an APS sensor, you need to use something closer to a 35mm lens. If you go even smaller, down to point-and-shoots, this is why you always find "35mm equivalent" thrown around. Because the sensors are so small, what's called, say, a 25mm wide zoom is really something like a true 4mm focal length, but functionally it acts like a 25mm lens on a full-frame camera.

Full-frames are beneficial for a few reasons. First, they're a lot less prone to noise. This is mainly due to their size. If you have three image sensors - a small point-and-shoot, an APS-size, and a full-frame, and they're all the same resolution...say 15MP...the pixels in the full-frame will be a whole lot larger, which equals a better ability to drink in light. Just like how it's a lot easier to capture fine detail in 1/32 than 1/72. Second, lenses act true to their focal lengths. Those, in my opinion, are the two biggest advantages.

On the question of depth of field...technically that's a myth. Depth of field is determined by three things, and three things only. The first is focal length - the distance from the focal point of the lens to the image sensor. The second is the distance to the subject. The third is the aperture.

Longer focal lengths, closer distances to the subject, and larger apertures = shallower depth of field.

Shorter focal lengths, farther distances to the subject, and smaller apertures = longer depth of field.

If you take a 50mm lens, and take a picture with the same settings from the same position with a full-frame and an APS camera, you will have the exact same depth of field. The difference is the field of view - which will be smaller in the APS sensor according to the crop factor.

But wait! If you adjust things so that your priority is keeping the field of view the same - by moving the APS-sensored camera further away, or putting a shorter lens on it, you're shooting the same image, but with a longer depth of field (because you increased distance and/or decreased focal length.

For model photography and other close-in work, a smaller sensor is actually better if you want to maintain a longer depth of field. Point-and-shoots are actually rather good at maintaining a long depth of field (and are conversely terrible at shallow DOF) because they operate at the extreme of this equation.

All of which I'm sure makes no sense whatsoever!
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DoogsATX
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Re: My camera

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JimD wrote:Wow. In English makes it a lot easier to understand. Thanks for taking the time to put that together.

So ideally I think full frame would be ideal overall...however...I just don't think a $1,500 camera is going to fly just so I can get pictures of models that I don't hate.
Yeah, full frame is overkill for just model photography. I use mine for a lot of family pictures as well...

Image

That and I've been drooling over a full-frame ever since the Nikon D700 came out back in 2008 or so. The D610 is one of the first where the price became one I was willing to pay.
So my understanding is if I were using a APS sized sensor and wanted to mimic your results I'd want an 80mm lens and a 100 mm lens (since you said you're using a 50 and 60mm...multiplying by 1.6...there isn't a 96mm lens I'm assuming)...and I'd need to be further from the subject to get the same DOF and FOV. (Assuming I'm understanding things correctly.
That's backwards, actually. To find the proper "focal match" for an APS-size sensor, divide by 1.6. So mimicking a 50mm lens would take you to a 31mm lens. My D300s packs a fixed 35mm lens, which is close enough and IMO the best lens you can get for APS-sized DSLRs. It's cheap, too!
Now increased noise is a drawback like you say...but realistically in the parameters of a set up for model photography is that going to be an issue....given a similar lighting setup.
No, noise won't be an issue. At least the way I shoot (ISO 100, f/22, then make the shutter speed whatever it needs to be). It's more if you're trying to shoot handheld, you can push into higher ISOs without getting grainy trash back. Came in really handy at my son's birthday party when my external flash decided to not work. This was shot at 5000 ISO:

Image
Are there any inherent drawbacks to variable lenses that would reduce image quality etc?
Personally, I'm not a fan of zoom lenses since the added glass can result in some reduced sharpness. That and they're never anywhere near as fast. I prefer fixed focal length/prime/whatever you want to call them lenses. Less glass, they're generally ultra-sharp, they're very fast, small, light. They're also almost always rather affordable, especially when you look at lenses designed for APS-sized sensors.
And one really dumb question....megapixels...seems intuitive that more is always better...but in the off chance I'll ask...go with as many as you can get? (Not that that seems to be cost prohibitive...it's just weird that I'm seeing DSLRs with lower megapixels pushing much higher prices than some with higher...though I'm sure there are other differences...but I'm looking at this on a very superficial level at the moment.)
Not a dumb question at all. More megapixels are not always better. In fact, for most use cases, they can actually be worse.

Here's the basic breakdown.

High Megapixel Count: Packs more (therefore smaller) pixels onto the image sensor.

Pros:
- Higher resolution image
- Better cropping (you can crop the stuffing out of a 24MP image and still have a 6MP shot to work with)

Cons:
- More prone to noise (smaller pixels aren't as good at drinking light, and more prone to noise as you bump ISO)
- Ginormous file sizes

Honestly, for DSLRs, higher megapixels aren't that bad. Point-and-shoots are the ones that really suffer...and on some I've seen noise become evident at like 600 ISO. Ridiculous.

Low Megapixel Count: Packs fewer pixels onto the sensor

Pros:
- Absolutely dominant low-light photography. The Nikon D700 (and D3) is still a legend in this regard. It had a 12.1MP full frame sensor...so the pixels were HUGE. You can basically use the thing handheld in the dark. You can push it to ridiculous ISOs - like 25000 ridiculous. This is admittedly useless in a studio setting...but out and about, when you can't control lighting, a lower MP camera is the way to go.

Cons:
- Cropping is more limited. A 12MP image will be half the size of a 24MP one. So if you crop it the same way, that 6MP image I mentioned above will instead be a 3MP image. Framing is a lot more important.

ALL THAT SAID, keep in mind that what we define as HD - 1920x1080 - is basically describing a 2MP image. A lot of this talk of megapixels and noise and resolution matters way more in photo processing and applying edits and such than it does to the final product - especially for posting things on the internet.
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Re: Sensors and Depth of Field

Post by Thomas_M »

Matt, I just dropped in here to say "Thank You!" for this supremely helpful and understandable contributions! Heartfelt thanks!

Oh, stunning and lovely pictures of your little ones! :)
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