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Forum: Troubleshooting and Beginner Help 09-07-2020, 12:58 PM  
Standard distance for autofocus fine tuning?
Posted By CarlosU
Replies: 39
Views: 1,378
That is right, but at least you can see the difference between using the recommended 30X FL distance and a shorter one.
Forum: Troubleshooting and Beginner Help 09-07-2020, 12:04 PM  
Standard distance for autofocus fine tuning?
Posted By CarlosU
Replies: 39
Views: 1,378
These are 100% crops from full size files I just took at 300mm and f4, the first at 15X FL distance (4.5 meters) and the second at 30X FL distance (9 meters). When I use the second image (9 meters distance) for the AF adjustment the differences between contiguous adjustment steps on the camera are almost imperceptible. That is why I prefer to use a shorter distance that gives a swallower DOF and consequently a more precise determination of the focus point.
Forum: Troubleshooting and Beginner Help 09-06-2020, 08:30 PM  
Standard distance for autofocus fine tuning?
Posted By CarlosU
Replies: 39
Views: 1,378
At least for me, with a DOF between 3 and 4 cms I still can estimate with some accuracy where the focus plane is, but beyond that I begin to guess the exact point of focus, which is crucial when doing the AF fine adjustment. The bigger the DOF the harder it is to determine where the exact focus plane is. That is using a plain flat focus target or a dedicated device like Datacolor Spyder Lenscal, which I have.

When I tried using the recommended 30 to 40 times FL distance I always ended with a DOF so big that most of the scale in the Datacolor device looked in focus.

I'll give you an example. I have a Sigma 100-300 f4 zoom. When adjusting AF at 300mm the recommended subject distance is 30 times the focal length, which is 9000mm or 9 meters. But with an APS-C sensor, a 300mm at f4 and a subject distance of 9 meters gives you aprox. 14cm of DOF. To me, that is almost useless to determine the exact focus plane. The same setting but at 4.5 meters distance gives a DOF of 3.4 cms, enough to see where the focus point is, and this is at half the recommended shooting distance.
Forum: Troubleshooting and Beginner Help 09-06-2020, 02:56 PM  
Standard distance for autofocus fine tuning?
Posted By CarlosU
Replies: 39
Views: 1,378
Depth of field (DOF) should be the variable to consider. You should do AF fine adjustment at maximum aperture so you have the smallest DOF possible. Then, you can calculate the DOF for your lens (lots of DOF calculators online) at maximum aperture and different distances. Then choose a distance that gives a DOF around 3 centimeters. More than that and it becomes difficult to pinpoint the focus plane.

For example, if you are adjusting AF for the 77 f1.8 you get a DOF of 3cm at a distance of 165cm, which is by the way 21 times the focal length of the lens.

For slower lenses it can be harder to determine the exact focus plane because of the larger DOF, and it becomes impractical to reduce the distance to achieve a swallower DOF.
Forum: Troubleshooting and Beginner Help 07-18-2020, 06:35 AM  
Is gap on KP ok, and is dust removal working?
Posted By CarlosU
Replies: 14
Views: 1,159
I read somewhere that the weather sealing on Pentax cameras (probably other brands too) is not 100% tight. It has some spots in the outer shell where tiny "valves" let air go in and out, but keep moisture and dust outside. This allows for changes in volume that occurs when zooming and focusing lenses. Maybe this gap is one of those spots?
Forum: Photographic Technique 07-15-2020, 02:56 PM  
Macro Why to reverse 50mm on extension tubes?
Posted By CarlosU
Replies: 12
Views: 794
Lens optical designs considers two distances: subject distance and image distance, the first from the optical center of the lens to the subject and the second from the optical center of the lens to the film/sensor plane.

On normal use, subject distance is much greater than image distance so lenses are designed for optimal results at those distances.

When doing macro at high magnification, 1X and greater, you increase image distance by using extension tubes or bellows, and at the same time reduce subject distance to a few centimeters. The image distance ends being greater than the subject distance, opposite to what the lens was designed for. In this case, using the lens inverted gives you better image quality than using it in its normal position.

To attach the lens to the extension tubes you use a reversal ring that has a filter thread on one side and a male bayonet mount on the other. You mount the ring on the filter thread of the lens so the lens ends with a bayonet mount on its front.
Forum: Troubleshooting and Beginner Help 07-11-2020, 12:31 PM  
What's wrong with my front element?
Posted By CarlosU
Replies: 19
Views: 1,274
I have seen this in some of my lenses. Sometimes almost imperceptible and other times quite obvious. I'm sure it is not fungus growth, which I also had to deal with in the past. Fungus growth is like irregular branches, while in this cases tiny spots distributed on a circular path.

My worst case was with a Tokina ATX 400mm 5.6. Images from the lens were foggy, with very low contrast. In all cases the haze seems to come from the inner surfaces so trying to clean front or rear of the lens was useless. I never tried to fix the problem with a professional service, this kind of service is almost nonexistent in my country or very expensive, and sending the lens abroad is very complicated and costly.

After some search and think I came to the conclusion that this is some form of degradation in the cement between elements. In the case of the 400, the lens was inside a car parked under the sun for a few hours before I noticed the problem. I've always thought that the heat affected (melted?) the cement in some way.

This is all speculation from my part but with support on some research and logic. Would like to know what others think.
Forum: General Photography 06-12-2020, 09:21 PM  
Why can i do macro without my reading glasses?
Posted By CarlosU
Replies: 6
Views: 665
I remember that in Modern Photography camera tests, among the specs they measure was the "apparent viewing distance" of the focusing screen, and if I remember well it use to be around a meter, depending on the tested camera.
Forum: Pentax SLR Lens Discussion 04-02-2020, 12:49 PM  
[fungus] open FA* 28-70 2.8 from rear
Posted By CarlosU
Replies: 7
Views: 662
You cannot avoid fungus spores get in to your lenses. They are practically everywhere. Anyway, they are minuscule and you can only see them through a microscope.
What you can do is prevent the spores to grow in to hyphae, the tiny filaments you see in the surface of a spoiled lens. Spores can only grow in to hyphae at humidity over 65%. I live in a very humid place, and after losing two lenses spoiled with fungus growth, I started to store my gear in a dry cabinet with lower than 50% RH. Never had fungus problems again. In normal use, the exposure to sunlight is enough to keep fungus growth under control. After using the camera and lenses, all the gear return to the cabinet. And if I will be shooting days away from home, I put some silica gel in the camera bag, and try to keep all the gear in dry and cool places as much as I can. As I said before, never had problems again.
Forum: Pentax SLR Lens Discussion 02-08-2020, 09:10 AM  
Pentax M 50mm f1.7 - stops per click
Posted By CarlosU
Replies: 3
Views: 408
2 to 2.8 is a full stop.

The click between 1.7 and 2.8 could be 2.4, which is a full stop from 1.7 and half stop from 2.8
Or could be 2, half stop from 1.7 but a full stop from 2.8

I used the lens for many years but in the eighties, more than 30 years ago, so I'm not quite sure.
Forum: General Photography 02-03-2020, 06:00 PM  
if you need some inspiration for the next phototrip - Morten Hilmer, Ellesmere Island
Posted By CarlosU
Replies: 11
Views: 780
Go figure!
Anyway, the logo on the video is almost identical to the Pentax camera brand logo, same color and same letters.
Forum: General Photography 02-03-2020, 04:25 PM  
if you need some inspiration for the next phototrip - Morten Hilmer, Ellesmere Island
Posted By CarlosU
Replies: 11
Views: 780
Anyone noticed the PENTAX logo in the first video? It's on the side of the boat at 3:05 mark. According to the narrator, the image should be from some years ago. Times when PENTAX sponsored arctic expeditions?
Forum: Pentax SLR Lens Discussion 11-14-2019, 03:25 PM  
DA20-40Ltd lens cap issue.
Posted By CarlosU
Replies: 33
Views: 1,515
I don't have the lens but I had an old vivitar zoom (70-150 if I remember well) that had the same type of cap. The old velvet lining lost friction and the cap fell constantly. I carefully remove the old velvet and replace it with a strip of self adhesive velvet flocking liner.
Forum: Pentax Compact Cameras 11-13-2019, 11:47 PM  
Pentax Zoom 90 WR - Want to learn more about it
Posted By CarlosU
Replies: 10
Views: 2,268
The 90 WR came with an IR remote that works perfectly with modern DSLRs. I had one, and I still use the remote with the K10, K5 and K3. Probably you have to put a fresh battery.
Forum: Pentax DSLR Discussion 11-05-2019, 10:18 AM  
Focus dilemma
Posted By CarlosU
Replies: 19
Views: 1,043
That's something I've been wondering since my first AF SLR, a Z1, and later with my DSLRs. Before that, it was a pleasure to manual focus with my MX in a big bright viewfinder with a very precise split image center and microprism ring.
When AF technology appeared all manufactures dropped the focusing aids from their focusing screens. No more split image or microprism aids, just a plain ground glass screen, and that makes MF very difficult in comparison. There exist some optional offers but not widespread.
I never knew the reason for that. Why not keep making the screens with focusing aids? Maybe manufacturers thought MF lenses would be replaced quickly and totally by AF lenses and users would never need to focus AF lenses manually?. I donīt know.
For a brand that bases a great part of its value on the possibility to use hundreds of MF lenses made in the last 40 years, It seems unexplainable why they drop the focusing aids on the focusing screens. The difference in manufacturing costs should be minimal.
After more than 30 years of AF technology, AF is still not perfect, specially for Pentax. I've lost shots because AF missed focus and I'm sure I could have nailed those same shots manual focusing on a screen with focus aids.
Forum: Pentax Film SLR Discussion 10-28-2019, 05:40 PM  
plastic film can - dumb question
Posted By CarlosU
Replies: 35
Views: 1,616
The cap on the translucent ones used by Fuji makes a perfect cap for the 40 limited hood. Much easier to put on and off and less worry if you lose one (as long as you have spares)
Forum: Pentax DSLR Discussion 10-11-2019, 12:10 AM  
Automatic Hyperfocal
Posted By CarlosU
Replies: 26
Views: 1,050
There is a line of Carl Zeiss lenses (Batis line, if I'm not wrong) that shows the exact focus distance and the DOF at the set aperture in an OLED display on the lens barrel. Maybe it also shows the hyperfocal, but not sure.
Forum: General Photography 10-06-2019, 07:23 PM  
With so many great offerings from Canon, Nikon, Sony, etc., why do you choose Pentax?
Posted By CarlosU
Replies: 107
Views: 3,624
I received my first Pentax (ME Super) as a gift in 1980. Today, more than 8 bodies and 16 lenses later (not all with me right now), I cannot justify to switch brand and the hassle that comes with it.
But if I was starting from scratch today, I'm not sure I would go with Pentax. I love what the brand gave to me all those years, and love the gear I have now. I'm so committed that I just bought a new 21 HD Limited last month. But nowadays the offer from other brands seems to be more right for first time buyers. Sure they will not get one of those "specialities" only Pentax can provide, but unless they need specifically one of them, they would make a safer decision going for other brand, in a broader sense.
Forum: General Photography 07-29-2019, 01:03 PM  
Environments favourable to Fungus
Posted By CarlosU
Replies: 7
Views: 701
I totally endorse this statement. I fought with fungus growth for a long time, even loosed two lenses that went beyond repair. The definitive solution came when I began to control principally humidity over other variables. I did this by storing my lenses and cameras inside a cabinet in which I put a "hot rod" (ceramic covered electric resistance) that gives a constant heat source to the inside, lowering humidity (low cost DIY solution, by the way).

I normally have values that go from 40% to 55% RH and 28°C to 32°C.
Initially I had some concern about the effect on mechanical parts (dry lubrication in bearings and helicoids, dryness on rubber rings and grips) but after some years using this method I have not seen any sign of problems and had zero fungus growth. That is living in a place where 75-85% RH is common around the year.
Forum: Photographic Technique 06-15-2019, 09:33 AM  
Macro Looking for a doodad thingy.....
Posted By CarlosU
Replies: 12
Views: 1,194
For table top macro work I use a base-flexible post-clamp device mounted on a mini lazy susan. Once the subject is positioned on the clamp I can rotate the set 360 degrees, so it is very easy to get the right position for the picture.
Forum: Photographic Technique 05-11-2019, 08:14 PM  
How much is enough - post processing
Posted By CarlosU
Replies: 30
Views: 2,179
Photography, as any other form of art, is a creative process. A pure and faithful reproduction of reality could be right but not necessarily. Depends on your intent.

For me (mainly nature photography) the goal is to trigger emotions in the observer comparable to what I felt when taking the picture. Sometimes this is done with an exact reproduction of reality but sometimes not. Sometimes you use special techniques or post-processing to emphasize some aspects of your subject that transmit a better message to the viewer. Sure you have the risk to look artificial or fakish, but at the end, at least for me, you are trying to convey feelings more than an exact copy of the reality in front of you.

Think of Van Gogh's "Starry Night". Far from an exact representation of a night sky, but a masterpiece that reflects what a tormented mind feels at a night sky.
Forum: Pentax SLR Lens Discussion 02-17-2019, 08:41 AM  
Size comparison: Pentax D-FA 100 WR macro vs. FA 100 macro
Posted By CarlosU
Replies: 60
Views: 5,455
I have had the FA since its introduction in 1992. Big, heavy sharp lens, and built like a tank. It is still in perfect condition.
I have been tempted by the size, weight and WR of the DFA, but always hesitated about the absence of a focus limiter (FL).
With the FA, when working at normal non-macro distances, the use of the FL makes a huge difference for AF.
If you don't use the FL, the lens sometimes focus all the way down to minimum distance and back to subject distance, which can be annoying. I have always suspected the DFA WR would do that as well but I'm not sure, does it?
Forum: Pentax SLR Lens Discussion 11-01-2018, 10:53 AM  
Sigma 100-300 F4 APO EX DG
Posted By CarlosU
Replies: 25
Views: 2,571
Shoot a fine detailed target in good light. Focus carefully, preferably in live view. Use a tripod and shoot with 2s delayed self timer. Shoot first at max aperture and then close one stop for each additional shot until f11. Examine the results at 100%. When I have some time today I can upload some 100% crops samples from my lens so you can see what kind of sharpness you could expect from the lens, at least from my copy.

On the contrary, what I meant was that your results with TC are on par with how this lens should perform with a TC.
This are quotes from Photozone site when they tested the lens in Nikon mount. Unfortunately they didn't do it in Pentax mount but the results should be the same.

" The Sigma AF 100-300mm f/4 EX DG HSM APO is one of the most impressive Sigma lenses tested to date. It is able to deliver a near-flawless performance with great resolution figures"

"The Sigma AF 100-300mm f/4 EX DG HSM APO delivered excellent resolution figures in the MTF lab matching the best zoom lenses in its range as well as many fix-focal length lenses"

"Combined with the Sigma AF 1.4x EX DG converter there's a drop in performance in the center but more so towards at the borders. However, the results remain generally on a very good level although it may be advisable to stop down a little here for an extra kick in terms of contrast."

Here the link for the complete test
Forum: Pentax SLR Lens Discussion 11-01-2018, 09:18 AM  
Sigma 100-300 F4 APO EX DG
Posted By CarlosU
Replies: 25
Views: 2,571
I don't have the Sigma TC but a Kenko 1.5 TC. That is the sharpness I would expect from my combination, which I seldom use by the way.
Since there is some loss of sharpness when using the TC, I would say your lens is OK. Do a test without the TC and that should give you a more clear idea of the lens quality.
Forum: Pentax SLR Lens Discussion 11-01-2018, 06:12 AM  
Sigma 100-300 F4 APO EX DG
Posted By CarlosU
Replies: 25
Views: 2,571
Are any of those pictures taken with the 1.4 TC?
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