Depth of field (1 Viewer)

[BoldenEagle]

Great information from several writers and sorry if I'm responding little late...

I was wondering dof mainly because I have two binoculars of which Zeiss Conquest 10x42 HD seems to have narrower dof than Swarovski SLC 10x56. I haven't done any of those tests that were introduced earlier in this thread but I can assure that when using Conquest, I have to use focusing wheel much more often than with SLC. That is, when observing migration, raptors etc. where most of the time I'm using binoculars to pick quite distant objects rather than going say from between 5 meters to 50 meters. Once I focus SLC to the horizon, I rarely have to focus again, unless something needs to be focused very near.

So in the field there is some difference between these two binoculars but based on the information given, I think the reason has to be something else than magnification or exit pupil (4,2 mm vs. 5.6 mm). It might be that Conquest has curved field or that my eyes accommodate easier with SLC? My Conquest is very sharp in the center of the field, in daylight totally comparable to SLC when mounted on a tripod and reading small text or using some other fine target. But towards the edges it softens more than SLC and also introduces more chromatic aberration.

Between scopes I have used, I haven't noticed any dof differences when used with same magnifications. I currently have Kowa Prominar 883 + 25-60x zoom and suffer with narrow fov @60x just because the scope has way too fast fine focusing. It would be ok if the fine focus would be about two times slower than it is, if using high magnifications.

It's true that you can't allways benefit high mags. because of bad seeing but when you have those good conditions, I think Kowa's 1.6x extender is a brilliant thing to have. In good conditions I can definitely see smaller details @96x than @60x without the extender. But fine focusing isn't certainly that easy...Also small exit pupil will have it's toll and the image is quite dim and not so easy to look.

I have lately been using Kowa TE-11WZ zoom a lot less than my APM 12.5mm Hi-FW eyepiece because with APM's 41x I can achieve a good compromise between enough magnification, exit pupil and dof. It's also sharper than my zoom and provides very wide fov and with cable tie sight I can easily find flying birds. Fine focusing with 41x is nearly ok...One of the few disadvantages is that I can't easily use higher mags because the only options are adding the extender or changing the eyepiece...APM also has quite obvious pincushion distortion but I'm quite tolerant to that because my SLC also has same distortions but I'm really not bothered by it.

Juhani

 

[Tringa45]

I have lately been using Kowa TE-11WZ zoom a lot less than my APM 12.5mm Hi-FW eyepiece because with APM's 41x I can achieve a good compromise between enough magnification, exit pupil and dof. It's also sharper than my zoom and provides very wide fov and with cable tie sight I can easily find flying birds.

Juhani,

Eyepieces may differ in FoV, field curvature, astigmatism, distortion and eye relief but, for terestrial applications, I don't think you are going to see any differences in central sharpness or contrast around 40x magnification.

Observed differences between scopes at the same magnification can usually be put down to the objectives.

Personally, I think the TE-11WZ is really excellent and the only minor criticism one could level is that it vignettes above 50x. However, you can really only see this with the eyepiece detached.

John

 

[BoldenEagle]

Eyepieces may differ in FoV, field curvature, astigmatism, distortion and eye relief but, for terestrial applications, I don't think you are going to see any differences in central sharpness or contrast around 40x magnification.

Observed differences between scopes at the same magnification can usually be put down to the objectives.

Personally, I think the TE-11WZ is really excellent and the only minor criticism one could level is that it vignettes above 50x. However, you can really only see this with the eyepiece detached.

I have done some resolution tests between three eyepieces I have for my Kowa: Baader Morpheus 12.5mm, APM Hi-FW 12.5mm and TE-11WZ zoom. I also have tested them all with the extender. I used Zeiss Diascope + 20-75 zoom as a reference. I can't see clear difference between Morpheus vs. TE-11WZ @65x; none of those two can resolve better than Diascope at corresponding magnifications and that difference is seen every time I have been testing it. APM resolves quite the same with Diascope; at least not less. I haven't got any "official" resolution test chart (just usaf 1951 test chart printed on ordinary paper and observed about distance of 35 meters) so I can't say how big the difference precisely is but at least this outcome has been very consistent as I have done it several times this way.

In practise it's not a big difference but I think it still is there. If I hadn't used reference scope, I'm not so sure the difference would have been so evident at all. As I have star tested my Kowa with zoom and compared it next to the Diascope and seen that Diascope performs better, I was little amazed that just by different eyepiece it can reach Diascope performance, because I have been told that eyepiece sample variation is usually very minimal.

I don't know if there is something wrong with my TE-11WZ sample but I nevertheless prefer the APM more, even if I can't zoom with it. I still have the zoom and mainly use it with the extender for those cases I need very high mags. Even if the zoom might not be perfect sample, with the extender it will give much more resolving power than APM, provided that seeing is good and no heat haze present. The zoom still offers very good overall IQ and it blows Diascope away with it's superior contrast and far easier use with eyeglasses but the APM is just even a bit more better imo.

In general, it's probably true that for birding it doesn't have to be perfect scope to make very detailed views but on the other hand, show me the person who is buying a scope and compares more than one sample and ultimately picks the scope which seems less sharp, even if the less sharp sample would already be a good sample...And the reality still is that even with so called "alpha" brands, you can have very poor sample and I have personally seen two so bad Kowa 883 samples that I would never have bought one of them, maybe some other will get one of those samples and will live happy with it. I myself used about 7 years Leica Apo Televid 82 which had bad undercorrection of SA and some astigmatism. When comparing side by side with other scopes @50x it still was better than many other samples.

Regards,

Juhani

 

[Tringa45]

I have done some resolution tests between three eyepieces I have for my Kowa: Baader Morpheus 12.5mm, APM Hi-FW 12.5mm and TE-11WZ zoom. I also have tested them all with the extender. I used Zeiss Diascope + 20-75 zoom as a reference. I can't see clear difference between Morpheus vs. TE-11WZ @65x; none of those two can resolve better than Diascope at corresponding magnifications and that difference is seen every time I have been testing it. APM resolves quite the same with Diascope; at least not less. I haven't got any "official" resolution test chart (just usaf 1951 test chart printed on ordinary paper and observed about distance of 35 meters) so I can't say how big the difference precisely is but at least this outcome has been very consistent as I have done it several times this way.

Juhani,

I doubt that a printed paper chart at 35 m is going to give you any conclusive results and unless you have exceptionally good eyesight, 65x magnification is not enough for resolution measurements.

For measurements on my Swarovski ATM 65HD and Kowa 883 I used a backlit 1951 USAF glass slide at 23 m and a 3,5 mm Televue Nagler for 131x and 143x magnification respectively. In group 2 they resolved elements 3 (5,04 line pairs/mm) and 5 (6,35 line pairs/mm). Even at 35 m your printed chart would have to show better than well defined 4 line pairs/mm - rather unlikely.

Again, I don't doubt that you see differences in your eyepieces, but as regards resolution, they are almost irrelevant. They just have to offer sufficient magnification (or be used with a booster) to allow you to see the resolution limits of the scope's objective.

Regards,
John

 

[BoldenEagle]

Again, I don't doubt that you see differences in your eyepieces, but as regards resolution, they are almost irrelevant. They just have to offer sufficient magnification (or be used with a booster) to allow you to see the resolution limits of the scope's objective.

You are right. I did some more precise observations (still not by as high standards as you have) by using 20€ bill details, again to rather see the difference between scopes/eyepieces, than try to measure the actual resolution.

However, I used a detail that contained about 5 line (not line pairs!) per millimeter. Distance was about 14 meters. With Diascope @65x I could see the lines (but couldn't count them) and see their orientation. With Kowa 883 I didn't see at least any significant difference between APM and TE-11WZ and I couldn't see there was distinct lines and their orientation was difficult to see @65x with both eyepieces. The difference between the two scopes must be (as you pointed out) resulting mostly of the fact that Diascope has little less aberrations (in fact, the star test revealed that only aberration is some SA unlike my Kowa which has also maybe slight astigmatism and just a tad of miscollimation and it also has distinct prism line).

It may be that the difference my eyes see at lower mags. is the enhanced contrast of APM. Also the TE-11WZ has good contrast but I think APM is even better. But it likely is that even superior contrast can't help resolving more if the image is not equally sharp because of more aberrations of the scope? Because the Diascope has very poor contrast but it will resolve smaller details anyway. Maybe the view with excellent contrast may seem sharper also between eyepieces and also at lower mags. using just black and white figures? In actual use I find my Kowa + APM be very sharp and somehow more pleasant than TE-11WZ (but I won't anymore say that it is sharper...). I still feel it gives somehow more clear image (among some other advantages).

I might try to measure the real resolution if I get an astro eyepiece with higher magnification because I can also use it with Diascope. And have to find some high quality resolution chart.

Regards,

Juhani

 

[Tringa45]

I might try to measure the real resolution if I get an astro eyepiece with higher magnification because I can also use it with Diascope. And have to find some high quality resolution chart.

Juhani,

As i mentioned, I used an Edmund Optics glass slide (about €80), but I see there is a cheap alternative, https://www.edmundoptics.de/f/pocket-usaf-optical-test-pattern/12147/, which would be quite OK if well illuminated.

The Kowa astro adapter with the external thumb screws limits the in-focus and a couple of my eyepieces (a 6 mm Vixen NLV and a 7,5 mm Baader Eudiascopic) can't achieve infinity focus. Finding something of short focal length that will is a bit of a lottery and I find the view at such small exit pupils unpleasant anyway.

Regards,
John