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11-05-2017, 08:41 AM   #1
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Full-frame lens on a crop body not advisable?

Tony Northrup posted a video a while back that I hadn't watched until just now. He makes a very reasoned argument against using full-frame lenses on crop sensors (with the exception of wildlife). He only mentions Canikon's but he is correct? This is the first I've heard of it, and if so should we Pentax users be opting for lenses designed with our crop sensors in mind rather than some of the full-frame glass? FWIW I have a lot of full-frame compatible glass myself.



Last edited by gatorguy; 11-05-2017 at 08:54 AM.
11-05-2017, 08:52 AM   #2
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I have two lens that are full frame and have been using them for several years without any issues.
11-05-2017, 08:58 AM - 1 Like   #3
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Very misleading: f/2.8 remains f/2.8 in terms of exposure.

Last edited by lytrytyr; 11-05-2017 at 09:11 AM.
11-05-2017, 08:58 AM   #4
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Does any of this make any sense?

11-05-2017, 09:03 AM   #5
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QuoteOriginally posted by gatorguy Quote
He makes a very reasoned argument against using full-frame lenses on crop sensors (with the exception of wildlife).
How about a summary? I'm finding the pattern on his shirt distractingly moire-inducing at the embedding videos size. I suspect he's trying to hypnotize me.

As a general statement, my DFA100mm and FA50/1.4 are parading around as counter-examples of lenses that are pretty darn happy on aps-c. So is my DA*300mm, but this might fall under the 'wildlife' exclusion.
11-05-2017, 09:07 AM   #6
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Aside from extra weight, as long as you know what you are doing there is no real downside to using a FF lens on a crop body.
11-05-2017, 09:09 AM   #7
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This sounds 'scientific' but many of the claims are misleading. He can stick to his Canikon crowd.

His first claim about multiplying the focal length AND aperture by the crop factor is wrong. He's obviously not going to explain what he means by it (or maybe he just doesn't know).

If I'm using the 70-200/2.8 on my KP, it behaves like a 105-300mm lens in terms of focal length and an f/4.2 lens in terms of effective DoF (compared to a FF 105-300mm lens). In terms of light gathering, it's still doing the same work as an f/2.8 lens does SO THERE'S NOTHING WRONG WITH USING IT. The bokeh will be as magnificent as if you're using it on the K-1, you're just not going to get as much of the background in your shot assuming the subject distance is the same in both examples.
11-05-2017, 09:11 AM   #8
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QuoteOriginally posted by BrianR Quote
How about a summary? I'm finding the pattern on his shirt distractingly moire-inducing at the embedding videos size. I suspect he's trying to hypnotize me.

As a general statement, my DFA100mm and FA50/1.4 are parading around as counter-examples of lenses that are pretty darn happy on aps-c. So is my DA*300mm, but this might fall under the 'wildlife' exclusion.
Essentially he's arguing (with backup evidence) that 2.8 on a full-frame lens ends up as something lower than that on a crop sensor since much of the light-gathering ability is wasted, unusable. I'm really waiting for one of our lens experts here to comment. I agree with some of the other posters that on the surface it doesn't sound right, but Northrup is not exactly a newbie either.

11-05-2017, 09:14 AM - 2 Likes   #9
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The truth is this. If you use a Full Frame lens on your crop body you are using a lens that has been designed for a larger image circle than you have a need for. This means the lens is larger than needed. It is also true the resolution of some older full frame lenses may not be as high as that of more modern crop (and full frame) lenses but that is of very little impact.

As for light gathering - as someone commented f/x.y is the same no matter what the lens image circle is. It is true you are "throwing away light" the light outside the sensor - but that has no impact on your exposure. The light per square mm is the same either way. The consequence of the light being thrown away is that the lens is heavier and larger than needed but that's all.

This same set of criteria is also in play when mounting lenses from full frame or crop on Micro 4/3 or from 645 to 35mm etc. The larger image circle lenses are just heavy and bulkier than the lenses made for the exact image circle needed.

The main benefits to using a larger image circle lens is sharing the lens over multiple systems or that corner performance and vignetting will be improved (typically) when using a lens designed for a larger image circle.
11-05-2017, 09:18 AM - 1 Like   #10
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If you want to compare DoF then you need to make sure your field of view is the same.

I can snap an image with a 200mm f/2.8 on the KP and the 300mm f/4 on the K-1 and the images should be very similar in terms of field of view and depth of field, as long as the subject is at a similar distance. If I then take the 200mm f/2.8 lens and put it on the K-1, in order to get a similar field of view I'll have to move forward. In this case the depth of field will be thinner as it is a function of subject distance, focal length and aperture.
11-05-2017, 09:21 AM   #11
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From what I've seen, tests, users, etc. my impression is that a FF lens is more likely to turn in lower vignetting and distortion overall than lenses cut just for APS-C, because the edges of the FF glass are out of the picture. Of course, then there's usually more size and weight involved, but sometimes this is offset as still the best choice, because of the flexibility of being usable on APS-C and both 35mm film and FF digital bodies.
11-05-2017, 09:28 AM   #12
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QuoteOriginally posted by gatorguy Quote
Essentially he's arguing (with backup evidence) that 2.8 on a full-frame lens ends up as something lower than that on a crop sensor since much of the light-gathering ability is wasted, unusable. I'm really waiting for one of our lens experts here to comment. I agree with some of the other posters that on the surface it doesn't sound right, but Northrup is not exactly a newbie either.
For most photographers the biggest attribute of Aperture is not light gathering ability but depth of field.

And I've said a bazillion times, You set the FF camera to F4, the APS-c camera to ƒ2.8 and the same ISO and Shutter speed collect the same amount of light. It's the same old hogwash folks like Tony have been publishing for years. They keep restating it with different takes, because they don't really understand it. Tony is the perfect example of a self proclaimed expert, that doesn't have the technical background to really understand a lot of the stuff he talks about.

You can't compare 2.8 on FF with 2.8 on APS_c because the DoF is different, they are different pictures. It's just a little tricky because most people assume 2.8 produces the same image on all systems. That one litlte difference seems to have turned 95% of the internet into disseminators of incorrect information.

It's a case of those who understand being yelled down by all the ignorant fools who assume they are right, because everyone else and the Tony Northrups of the world don't understand either, so as far as they are concerned the consensus of the internet is on their side and folks who say different are wrong.

That Tony would still be posting this nonsense is completely disappointing.

An FF lens used on APS-c produces twice the size of image circle it needs to, but that is not a disadvantage photographically speaking. The only possible disadvantage would be if the APS-c camera didn't have internal baffles to block the extra light and keep it from reflecting around in the inside of the camera body.

Looking at the inside of my K-3 and my wife's K-5, I see the necessary ballers. These cameras were designed to us FF glass. Something Tony neglects to mention.

Tony makes money from his website, spreading his version of photo reality, none of us get paid for correcting him.That the way of the world. If you can come up with a logical sounding half truth that draws people to your webpage, people will send you money and watch your videos. The truth is so boring (and obvious) , nobody ever thinks they should send you money.

If you benefitted from this explanation send 5 bucks to the room.

I'm partly kidding... we are volunteers here.

But partly not kidding, it cost to keep the site running.

Last edited by normhead; 11-05-2017 at 09:47 AM.
11-05-2017, 10:09 AM - 1 Like   #13
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I happily use the D FA28-105mm on the K7, and I think anyone would be hard pressed to find any disadvantages to it. Indeed, one big advantage to it is that I won't need to buy another wide to mid-tele zoom if I eventually get a FF Pentax camera, something that figured in my reasoning when I bought the lens.
11-05-2017, 10:15 AM - 5 Likes   #14
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QuoteOriginally posted by gatorguy Quote
Essentially he's arguing (with backup evidence) that 2.8 on a full-frame lens ends up as something lower than that on a crop sensor since much of the light-gathering ability is wasted, unusable. I'm really waiting for one of our lens experts here to comment. I agree with some of the other posters that on the surface it doesn't sound right, but Northrup is not exactly a newbie either.
Tony Northrup is full of crap.

Your lenses have f stops and t stops. F stops are a physical character of the lens. IE exactly how big is the aperture.

T stops measure how much light the lens can transmit from the front of it to the sensor. This is more a measure of efficiency of the design.

A full frame lens will transmit whatever amount of light that it is able to do.

The problem and mistake in his theory is that they are measuring at the sensor.

A smaller sensor will naturally record less light volume wise sheerly because of size.

A bigger sensor will naturally record more light because it's bigger.

There is a big flaw in his theory and methods.

If you put a 5 gallon bucket in your back yard and right next to it you put a kiddie swimming pool.

When it starts to rain which one will capture more rain? Naturally the kiddie pool will have a larger volume of water in it.

Tony Northrup on the other hand is asserting that this is because it it rained less over the top of the 5 gallon bucket.
11-05-2017, 10:17 AM   #15
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About half of my lenses are full frame and I have never had a problem with them using them on an APS c body.
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