Wednesday, February 26, 2014

How much should you Spend on Glass for APS-C sensor cameras?

Good Wednesday from Prospect.

To begin with, sorry I've been trying to stay off as much social media as I can lately and just honing my skills, and getting food on the table. Seems to take much more effort as of late for the same burger.

Canon 5D mkII, Canon 70-200 II f2.8 at 165mm, f2.8, 1/2500
Anyway, todays post I want to talk about full frame vs APS-C sensors as they relate to composition and DOF. Yes both of these play a big role in what gear we shoot and frankly we'll find that although APS-C cameras are relatively less expensive than FF we'll see we'll need to spend more on glass the smaller the sensor. Unless of course all you want is everything in focus all the time (point and shoot your heart out).

This post will not touch on light gathering capabilities between full frame and crop sensors (APS-C) or their pixel size and counts (too much tech there thanks). As sensors mature that really doesn't have as much bearing as it used to. But sensor/film size will always have an effect of DOF and BOKEH.

As we explain the effect of depth of field between the two we will see a trend that will explain why the crop glass should be the fastest we can afford.

Bare in mind this isn't really a tech post as so much as a technique post. What helps us to get that great BOKEH or control over the subject matter along with the composition. I am, also, not judging which is better, FF or APS-C (crop sensor), just the differences we need to be aware of.

First let me explain, briefly, what depth of field (DOF) is and what BOKEH is, for those who are unfamiliar.

Simply put DOF is that part of the subject that is in focus. To start, when we focus on a subject 1/3rd (33%) of the space in front and 2/3rds (67%) the space behind are included as in focus. 

The factors that determine that are:
 1 - the focal length of the lens
 2 - the aperture we are shooting at
 and
 3 - the distance from the camera to subject.

Fuji XF 14mm f2.8 with dof scale
Most high-end and manual focus lenses are imprinted with a scale that express what that lens takes into focus at any chosen aperture and distance. If you don't have that at hand here's a great resource to figure it out: http://www.dofmaster.com/doftable.html

(Oh BTW there's also an app for that, linked on the same page to the left. I have my copy.)

BOKEH is all that out of focus bit in front and behind the subject.

The combination of DOF and BOKEH are what helps, or hinders, the final composition.

So here is where cameras with different sensor sizes can change how the composition will look.

When we are shooting with either FF or APS-C, size does matter. A full frame sensor is typically Canon: 35.8 mm w x 23.9 mm h, where as an APS-C is typically 22.2mm w x 14.8mm h (Nikon: FF  36mm w x 23.9mm h, APS-C is typically 23.6mm w x 15.7mm h). It varies from model to model but a crop factor of 1:1.5 is common (50% smaller than FF). So the distance to subject, on a cropped sensor, will be about 50% further away from the subject to frame the same composition. 

Here's the story everyone hears at the camera store when it comes to crop sensor cameras: CAMERA SALESMAN: "This is a 50mm lens but on the APS-C camera it's about a 75/80mm lens, great for portraits, or that 35mm will make a great standard 50/56mm., or a 70-200mm is really a 105-300mm, great for sports." Insert whichever you wish in that conversation.

What no one wants to explain is what effect that has on the final composition and how it effects DOF and BOKEH. That falls in the full frame/crop sensor sales pitch.

So if by comparison we are always standing 50 percent further back to achieve the same crop with an APS-C size sensor than that's going to ultimately effect the depth of field (DOF), especially if we want the same amount of subject in focus. Now some will say that's a benefit. I'm not saying one has an advantage over the other. I'm just saying it effects how we shoot.

Canon 5d mkII, Canon 85mm f1.8 at ISO 200, f8, 1/125th
I wanted her face and hair in focus but her neck and shoulders to
fall away with subtle softening at the ears
The one thing I dislike is fuzzy hair if I can help it.
But here's my point about more expensive glass on APS-C cameras.

Theoretically every shot we make at f2.8 with a full frame camera, we will need to shoot at 50 percent more open to achieve the same thing because we're 50% further from our subject i.e.: f2 for the same effect. f4 would be f2.8., f5.6  would be f4 and so on (one full stop). 

Remember the percentage difference from front to back DOF? Here's a simple explanation.

Lets say the subject is 10 feet from us with a FF camera, same crop will set us at 15 feet (50% further away) And suppose that at FF - f4 we achieve 1 foot in front and 2 feet behind for subject in focus(3 feet in total). With an APS-C we will get 1.5 feet in front and 3 feet behind, 50% more (4.5 feet in total). To narrow down the same DOF and achieve the same BOKEH we will need to open the aperture to 2.8 (1 stop). So right off the bat we need faster glass to achieve the same effect. (Note: what to do if we're shooting 2.8 FF?)

But you say hey we can use a different focal length on an APS-C camera to be equivalent to that FF. Ahh, not so fast, now we have to compare the differences in compression as it relates to different focal lengths. The higher we go in focal length the more compressed a subject will look (more 2 Dimensional than 3 Dimensional)

By using say a 35mm on an APS-C may indeed crop the same and place us the same distance as a 50mm lens on FF but those lenses are very different when comparing compression, which is a crucial part of DOF. We can make the comparisons on the chart at this link (http://www.dofmaster.com/doftable.html) listed above as well. 

Here's an example, both resulting in the same crop/composition: 
(Note the Canon APS-C is actually set at 1.6 crop factor)

35mm (56mm with 1.6 crop) on a Canon 7D (APS-C) at f4 produces, at 10 feet from subject, 
a distance of 8' 5.1" - 12' 4"in focus (approx 4' 11")

50mm on a Canon 5D (FF) at f4 produces, at 10 feet from subject, 
a distance of 8' 8.9" - 11' 8" in focus (approx 2' 11")

To achieve the same DOF the 7D would need to stop up to f2.8  for 8' 10" - 11' 6" (approx 2' 8")
(BTW 35mm on a FF at f4, 10 feet is 7' 8.7'' - 14' 2'' (approx 6' 5") so it does have an effect, just not enough.)

Using the same focal length on both cameras to achieve the similar DOF is tough but here it is (approx). Note: the framing will be different between the shots at 10 feet and similar at 15 feet.

50mm (80mm with 1.6 crop) on a Canon 7D (APS-C) at f5.6 produces, at 10 feet from subject a distance of 
8' 10.3'' - 11' 6''in focus (approx 3' 1") (cropped composition-slightly deeper DOF)

50mm (80mm with 1.6 crop) on a Canon 7D (APS-C) at f4 produces, at 10 feet from subject a distance of 
9' 2"' - 11' in focus (approx 1' 10") (cropped composition-shallower DOF)

50mm (80mm with 1.6 crop) on a Canon 7D (APS-C) at f4 produces, at 15 feet from subject a distance of 
13' 2.4'' - 17' 4.8'' in focus (approx 4' 2.4") (same composition-deeper DOF)

50mm (80mm with 1.6 crop) on a Canon 7D (APS-C) at f2.8 produces, at 15 feet from subject a distance of 
13' 8.4'' - 16' 7.2''  in focus (approx 2' 11") (same composition-similar DOF)

50mm on a Canon 5D (FF) at f4 produces, at 10 feet from subject a distance of 
8' 8.9" - 11' 8" in focus (approx 2' 11") (same composition-similar DOF)

As we compare we can also see a trend of a very different nature in the in-front and back focus ratio. Sorry I'm not going to get into Hyperfocal distance (Great article here http://www.dofmaster.com/hyperfocal.html) or circles of confusion. My goal is to simply show the differences between the two and how they relate to the overall composition and technique.

By the way the faster the glass (more open aperture ability) the more expensive it is, incase for some inexplicable reason you didn't know. ( to illuatrate the difference, a Leica 50mm f1.4 Summilux goes for about 4000.00us and the most expensive 50mm lens we know of for full frame is a Leica 50mm f0.95 Noctilux for 11,000.00us. In DSLR standards, a Canon 50mm f1.4 is about 400.00us and a Canon 50mm f1.2 is 1620.00us. All 35mm film/sensor sizes. I haven't seen a dslr lens at 0.95 yet and I don't think it can be done given the lens to film plane distance, at a decent price anyway.

It's always mentioned at how creamy the BOKEH is on full frame and Medium format. When we boil all this down it simply shows that if you are going to buy APS-C sensor cameras, and want that same great look, you need to spend even more on the glass. Yup f1.2 and f1.4 is in your future or at least f2.8 as a minimum.

As a side note, I do tote a Fuji XPro-1 when I walk around shooting candid and street work. My glass starts at f2.8 on all my cameras but I tend to go faster on this XPro-1 (f1.4 and f1.2 is the norm and primes only) If you're not familiar with that camera it's APS-C. I chose that for the size of gear to carry and the results in the environment I intend to use it. You can see the results throughout this blog.

Now if that XPro-1 or fuji X equivalent, XPro-2 maybe, was full frame I'd drop my big DSLRs in a heart beat, or at least when I add digital medium format, I shoot film with that format currently. Which by the way achieves even creamier BOKEH at higher apertures. You see my goal is all about the sharpest image with the best control over DOF to blow a background out at a sharp aperture, say 6.3 to 8 and still get great BOKEH. That's where full frame and medium format come in.

One more note, the case could be made that APS-C is better for landscape because you get a longer DOF but as reality would have it the further away we are from a subject the more, in focus it is anyway.

Enjoy, Derek

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