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07-10-2013, 07:48 AM   #16
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QuoteOriginally posted by GeneV Quote
Time to get out the old mirror lens.
I wasn't happy with a Sigma 150-500, so I doubt a mirror lens would satisfy me.

My Pentax birding system is not fully implementable yet. I have a DA*300 f4, which is sharp enough to withstand 100% crops on a 16mp body, even with a Kenko 1.5X TC attached. The problem with the Kenko is auto-focus. If the target is a sitting duck, it's slow to focus and the focal point is sometimes random. For action, fuggedaboutit!

I need the roadmapped Pentax SDM 1.4X TC, the sooner the better. A 24mp, APS-C sensored, pro-spec body, with improved AF will be the finishing touch. I'm hoping my wish list will be available this fall, but I've been disappointed in the past, more than once.

07-10-2013, 07:01 PM   #17
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QuoteOriginally posted by audiobomber Quote
I wasn't happy with a Sigma 150-500, so I doubt a mirror lens would satisfy me.

My Pentax birding system is not fully implementable yet. I have a DA*300 f4, which is sharp enough to withstand 100% crops on a 16mp body, even with a Kenko 1.5X TC attached. The problem with the Kenko is auto-focus. If the target is a sitting duck, it's slow to focus and the focal point is sometimes random. For action, fuggedaboutit!

I need the roadmapped Pentax SDM 1.4X TC, the sooner the better. A 24mp, APS-C sensored, pro-spec body, with improved AF will be the finishing touch. I'm hoping my wish list will be available this fall, but I've been disappointed in the past, more than once.
Sounds like what you really need is an extra $5-6k and you'd be set with a nice 500mm+ prime.

If you get past the fixed aperture, the old Tamron 500mm does a pretty nice job, and it carries up mountains well.

Last edited by GeneV; 07-10-2013 at 07:14 PM.
07-11-2013, 01:59 AM   #18
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QuoteOriginally posted by Class A Quote
If you are using an FF-equivalent focal length then you need to convert the f-ratio as well.

The lens on the Q is equivalent to a 275mm f/7.7 on FF.
Only partially correct
Just as a 50mm lens is a 50mm lens irrelevant of sensor format so its true a f1.4 lens is also and f1.4 lens irrelevant of format

so a true statement would be
a 50mm f1.4 lens on a 5.5 crop factor camera gives and equivalent 12mp ff 275mm f1.4 with the DoF of an f7.7
As Aperture is usually about light gathering rather than DoF it more correct IMO to state the 1.4 capability than the increased DoF as an F factor.
07-11-2013, 05:07 AM   #19
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QuoteOriginally posted by awaldram Quote
so a true statement would be
a 50mm f1.4 lens on a 5.5 crop factor camera gives and equivalent 12mp ff 275mm f1.4 with the DoF of an f7.7
As Aperture is usually about light gathering rather than DoF it more correct IMO to state the 1.4 capability than the increased DoF as an F factor.
Here's a true statement, a 50mm lens mounted on a Q and set for f1.4 is dead soft and throws more purple fringe than anything you've ever seen. Anyway, we should get off the Q and back on topic.

The OP was not asking about a Q.


Last edited by audiobomber; 07-11-2013 at 07:56 AM.
07-11-2013, 05:58 AM - 1 Like   #20
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QuoteOriginally posted by awaldram Quote
Only partially correct.
No, my statement was a 100% correct.

If you want to make an "apples to apples" comparison, you need to talk about image quality.
Image quality is determined by the total light gathered for an image. It is not determined by "exposure" which is defined as relative flux.

It is incorrect to separate the notions of "light gathering" and "depth of field". The latter becomes more shallow when the former becomes higher and vice versa. Any additionally light gathered blurs the image more in the OOF areas.

One useful approach to think about equivalent focal lengths and f-ratios is to view a teleconverter as a format converter. A 1.5x teleconverter converts every APS-C lens into an equivalent FF lens. The focal length is increased by the factor 1.5 and the smallest f-ratio is as well.

Would you argue that when you add a teleconverter to a lens that the DOF changes, but the light gathering capability does not?
07-11-2013, 07:30 AM   #21
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QuoteOriginally posted by Class A Quote

Would you argue that when you add a teleconverter to a lens that the DOF changes, but the light gathering capability does not?
I think your getting a little confused your point has no bearing on sensor size or F ratio.

A teleconverter alters the F ratio because of the apparent aperture Vs magnification causes a reduction in light ie. 2x converter reduces light by 4 time or 2 stops as the aperture appears half it original size due to the magnification.

This is easily proved as a front mounted converter because it has no affect on apparent aperture also has no affect on F ratio

i.e a 50mm f1.4 lens with a 2x front converter delivers 100mm f1.4 whereas with a rear converter would be 100mm f2.8

A different sized sensor has no affect on the lens apparent aperture so again the f ratio remains constant.
07-11-2013, 07:42 AM - 1 Like   #22
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QuoteOriginally posted by awaldram Quote
I think your getting a little confused ...
I'm not getting confused.

If you want to get rid of the misconception that there are two unrelated notions of "light gathering" f-ratio vs "DOF" f-ratio, the following resources may help:
1. Digital Camera Sensor Sizes: How Do These Influence Photography?
2. The true reasons for a full frame camera.
3. Equivalence.

I'll make one last attempt to explain why I referred to a teleconverter.

If you have a given lens, say a 50/1.4 that creates an FF image circle, it does not matter whether you crop into that image circle with an APS-C sensor, or you keep an FF sensor but "crop" by enlarging the image circle in such a way that only the APS-C portion of the original image is projected on all of the FF sensor. In both cases, you achieve higher magnification (less AOV), you change the DOF, and you disregard some light, i.e., you change the "light gathering". In both cases, you lose the light that the lens gathered but is projected outside the sensor.

So if you agree that a teleconverter not only affects the focal length of a lens, but also its light gathering capability (in terms of what is captured by the sensor), then you must also accept that using a smaller sensor not only changes the FOV (resulting in a different equivalent focal length), but also changes the light gathered (in terms of an equivalent f-ratio).

Last edited by Class A; 07-11-2013 at 07:55 AM.
07-11-2013, 07:49 AM   #23
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To the OP - It is going to be a tough call. Of course you are on a Pentax forum, so we all love our cameras. I loved the design on the K5 and so I was happy to move to a K5IIs. Everything stayed in place and the AF improves. Plus, the sharpness from the lack of AA filter is sometimes blinding! It is a great camera.

Still, changing systems is a big deal. Canon does offer more options to you, so you may need to consider what you want from your camera system 3-5 years from now...but those options may ultimately cost you more. Despite the rise in costs over the past year or so, Pentax does offer a good value when compared to other systems.

The final thing to consider is that everyone is anticipating that the IIs will be replaced with a new flagship Pentax in the fall. Unless you are planning some major trip or event where you will be doing birding, you might want to wait. A 24mp aps-c with even further improved AF might be exactly what you are looking for.

07-11-2013, 09:05 AM   #24
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QuoteOriginally posted by Class A Quote
I'm not getting confused.

If you want to get rid of the misconception that there are two unrelated notions of "light gathering" f-ratio vs "DOF" f-ratio, the following resources may help:
1. Digital Camera Sensor Sizes: How Do These Influence Photography?
2. The true reasons for a full frame camera.
3. Equivalence.

I'll make one last attempt to explain why I referred to a teleconverter.

If you have a given lens, say a 50/1.4 that creates an FF image circle, it does not matter whether you crop into that image circle with an APS-C sensor, or you keep an FF sensor but "crop" by enlarging the image circle in such a way that only the APS-C portion of the original image is projected on all of the FF sensor. In both cases, you achieve higher magnification (less AOV), you change the DOF, and you disregard some light, i.e., you change the "light gathering". In both cases, you lose the light that the lens gathered but is projected outside the sensor.

So if you agree that a teleconverter not only affects the focal length of a lens, but also its light gathering capability (in terms of what is captured by the sensor), then you must also accept that using a smaller sensor not only changes the FOV (resulting in a different equivalent focal length), but also changes the light gathered (in terms of an equivalent f-ratio).
I understand what your saying but totally disagree, If you cut a segment out of a FF 50 f1.4 image it will still be correctly exposed.
AoV does not alter a 50mm lens remains a 50mm lens it just appears to offer more reach due to greater pixel density and closer equivelant viewing the same reason the DoF appears reduced. It not reduced in reality.
Let put the theory to the test
Both show the same exposure taken in auto @iso200 f2.8 both camera selected a shutter of 1/3200

therefore the same aperture but one looks like 60mm f2.8 the other 220mm @ f2.8
The exposure certainly has no resemblance to 220mm @f16




Last edited by awaldram; 07-11-2013 at 09:11 AM.
07-11-2013, 12:30 PM - 1 Like   #25
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QuoteOriginally posted by blurwi Quote
Hope this makes sense.
Well, it doesn't.
And welcome to the forum

I think you ought to research requirements for photographing birds, find the appropriate tools, and then get up the budget, saving if necessary. How serious of a bird photographer do you consider yourself to be? And are the birds you photograph hanging 'round a birdfeeder (easy pickins' for any camera) or flying around in the wild?

If you are fairly serious, meaning that you don't shoot many other subjects and you seek out bird shooting opportunities regularly, then the 6D is not a good choice, its AF is too slow as is the processing power. Pentax shouldn't be part of the conversation either as the AF is subpar and the long lens choices are on the dismal side. I dropped my Pentax K20D for birds and sports four years ago for a 7D and consider it one of the smarter business calls I've made. Pentax is great for travel and art let me proclaim.

Consider selling your 6D and spending the proceeds towards a 7D and say the 100-400L lens. 7Ds used or refurbed have come down in price. I'm sure the next MK2 version will be kickass, but even after 4 years, the 7D is still the highest performing APS-C sensor camera available new. The sensor is inferior to what the K-5 series offers, but for birds and sports that is a lesser requirement. The camera is very well built; I've hauled it on my back for many miles, shlepped it to dozens of sports facilities, shot in the rain and hail and snow with zero complaints. Since you own a Canon DSLR already, you've mastered the hard part being the less-than-wonderful user interface, though the 7D is as good as Canon gets.

Hope this helps.

M
07-11-2013, 06:14 PM - 1 Like   #26
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QuoteOriginally posted by awaldram Quote
therefore the same aperture but one looks like 60mm f2.8 the other 220mm @ f2.8
The exposure certainly has no resemblance to 220mm @f16
  1. It is not the same aperture. It is just the same f-ratio, leading to different aperture diameters for different focal lengths.
  2. If you change the focal length from 60mm to 220mm then the respective f-ratio changes from f/2.8 to f/10, not to f/16.
  3. The cropped image has the same image quality that you'd get from using a 220mm lens on the larger sensor, stopped down to f/10.
As I said in the beginning what matters for image quality is the total amount of light (number of photons) captured, not the exposure.

Exposure tells you about the amount of light for a unit area. It is a good measure for the light coming in, but not at all for the light recorded. Let's assume the same f/2.8 equivalent exposure for two sensors. One has the size 1mm x 1mm and the other has the size 24mm x 36mm. The latter collects 864 times more photons than the smaller one. The difference corresponds 9.75 stops. Let's assume that the same part of a scene is projected on both sensors (which is the only way to do an "apples to apples" comparison). Are you telling me that you don't mind which image you get; the one that uses say 1000 photons or the one that uses 864,000 photons, just because you can argue that both got the same "exposure"?

The above is an argument why looking at f-ratios that correspond to "exposure" is not helpful when talking about image quality. Another argument is that the widest f-stop for a lens is simply its focal length divided by its aperture diameter. If you calculate FF-equivalent parameters, you need to multiply the focal length by the crop factor. In order to get the same DOF, the FF-equivalent lens needs to maintain the same aperture diameter, hence the widest f-stop of the FF-equivalent lens will be crop_factor*original_focal_length/aperture_diameter which is the same as crop_factor*original_widest_f-stop. End of story. Exposure (in the sense of the amount of light on a unit area) doesn't play into it at all.

Last edited by Class A; 07-11-2013 at 06:20 PM.
07-11-2013, 10:13 PM   #27
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QuoteOriginally posted by Class A Quote
  1. It is not the same aperture. It is just the same f-ratio, leading to different aperture diameters for different focal lengths.
  2. If you change the focal length from 60mm to 220mm then the respective f-ratio changes from f/2.8 to f/10, not to f/16.
  3. The cropped image has the same image quality that you'd get from using a 220mm lens on the larger sensor, stopped down to f/10.
As I said in the beginning what matters for image quality is the total amount of light (number of photons) captured, not the exposure.

Exposure tells you about the amount of light for a unit area. It is a good measure for the light coming in, but not at all for the light recorded. Let's assume the same f/2.8 equivalent exposure for two sensors. One has the size 1mm x 1mm and the other has the size 24mm x 36mm. The latter collects 864 times more photons than the smaller one. The difference corresponds 9.75 stops. Let's assume that the same part of a scene is projected on both sensors (which is the only way to do an "apples to apples" comparison). Are you telling me that you don't mind which image you get; the one that uses say 1000 photons or the one that uses 864,000 photons, just because you can argue that both got the same "exposure"?

The above is an argument why looking at f-ratios that correspond to "exposure" is not helpful when talking about image quality. Another argument is that the widest f-stop for a lens is simply its focal length divided by its aperture diameter. If you calculate FF-equivalent parameters, you need to multiply the focal length by the crop factor. In order to get the same DOF, the FF-equivalent lens needs to maintain the same aperture diameter, hence the widest f-stop of the FF-equivalent lens will be crop_factor*original_focal_length/aperture_diameter which is the same as crop_factor*original_widest_f-stop. End of story. Exposure (in the sense of the amount of light on a unit area) doesn't play into it at all.
In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography.
You can see from the statement above that you first point is complete balderdash relative aperture and F-ratio are the same thing so saying one is different yet the other the same is contradicting yourself.

I fail to understand how you can think that putting the same lens with the same registration distance can suddenly change its aperture it is just ludicrous F-stop is a fixed number based on lens physical length and diameter of entrance pupil.
I've proved it in the images above if you wish I can add the teleconverter image and you'll see the shutter speed drop and the entrance pupil shrinks.

Let me say again
F-stop is the ratio of the lens's focal length to the diameter of the entrance pupil It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography.

I've highlighted the important piece in this discussion,
The only thing that alters in a crop sensor is the perceived focal length due to the reduced angle of view, DoF , exposure etc all remain the same as the lens would deliver on any format camera
You could crop the identical image of the Q above from the K5 image below it and they would be identical (if lower resolution) your suggesting that once I've cropped the image the effective F-stop changes to f10 ? !!!!! Juts complete madness IMO

As for Image quality (how you think that's related to sensor size I don't know) it is a factor DR, resolution micro lens accuracy, noise floor , lens quality well depth etc etc nothing related with sensor size .
Again I think your confused by the observation larger sensors tend to deliver higher IQ at the same settings, This is because they tend to have better heat dissipation (all else being equal resolution etc) leading to lower noise floor.

Last edited by awaldram; 07-11-2013 at 10:31 PM.
07-11-2013, 10:53 PM   #28
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QuoteOriginally posted by awaldram Quote
You can see from the statement above that you first point is complete balderdash relative aperture and F-ratio are the same thing so saying one is different yet the other the same is contradicting yourself.
You did not write "relative aperture". You wrote "aperture". The latter refers to an opening with a diameter. The term "f-ratio" refers to the quotient of focal_length/aperture_diameter.

You are now arguing that your interchangeable use of "f-ratio" and "aperture" was correct and that I don't know what I'm talking about. You are suggesting that I've made all sorts of statements that I didn't. For instance, I have never suggested that the widest f-stop of a lens changes. I have always talked about "equivalent lenses".

This has become too tedious to continue. You have the right to keep your misconceptions and I see you are exercising it.

You don't have to believe me. If my explanations have not been lucid enough, I suggest that you try the three sources I've referenced earlier. If you want to ignore these as well, again, you have the right to do that.

I'm out of here.
07-11-2013, 11:27 PM   #29
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You are correct its now pretty tedious so I too will withdraw.
I'll keep my science you keep your hocus pocus (see we can all use empty words )
As to your references two do not contradict anything I've said and the third is from a source I've often decried as incorrect and more about self publicity than fact.
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